What is angiosperm?

• Within the kingdom Plantae, Angiosperms are the flowering plants and they are the most diverse group with largest number.
• Angiosperms make up nearly 80% of all recognized green plants that now live.
• Angiosperms dominate the surface and vegetation of the Earth in more ecosystems than any other group of plants, especially terrestrial habitats.
• Birds and mammals including humans rely on Angiosperms, as it is the ultimate source for the food.
• Furthermore, the most economically significant group of green plants are flowering plants, which serve as a source of pharmaceuticals, fiber products, timber, ornaments and other commercial products.
• Although the angiosperm taxonomy is still not well understood, a broad body of comparative data derived from DNA sequence studies is included in the new classification scheme.
• It is identified as the Angiosperm Phylogeny Group IV (APG IV) botanical classification system.
• Angiosperms is known to be a category called Anthophyta at the level of division (comparable to the phylum level in animal classification systems), however the APG system only identifies informal groups above the level of order.
• The number of forms found among angiosperms is larger than that of any other group of plants.
• The size ranges from the smallest individual flowering plant, possibly the watermeal (Wolffia) which is less than 2 millimeters (0.08 inch), to one of the largest angiosperms, Australia’s mountain ash tree (Eucalyptus regnans) at around 100 meters.
• Angiosperms of nearly every size and form lie between these two extremes.
• Succulent cacti (Cactaceae), delicate orchids (Orchidaceae), baobabs (Adansonia species; Malvaceae), vines, rosette plants (Asteraceae) and carnivorous plants such as sundews (Drosera; Droseraceae) and Venus flytrap(Dionaea muscipula) are examples of this variability.
• It is important to consider the basic structural plan of the angiosperms to understand this vast array of types.
• The fundamental forms of angiosperms are woody or herbaceous.
• Woody forms(generally trees and shrubs) are abundant in secondary tissues, while herbaceous forms (herbs) seldom have any.
• The herbs that complete their growing cycle within the same season are annuals.
• Cultivated garden plants, like maize (Zea mays; Poaceae), beans (Phaseolus and other genera; Fabaceae) and squashes (Cucurbita; Cucurbitaceae), along with wildflowers such as some butterflies (Ranunculus; Ranunculaceae) and poppies, are the examples of annuals.
• Biennials are also herbs, but their growing period, unlike annuals, lasts two years: during the first year, vegetative (non-reproductive) plant growth occurs from seed, and during the second, development of the flowers and fruit takes place.
• Well-known biennials include the beet (Beta vulgaris; Amaranthaceae) and the carrot (Daucus carota; Apiaceae).
• A perennial grows for several years and mostly flowers yearly.
• At the end of each growing season, the aerial parts of a perennial die back to the ground in temperate regions and new shoots from such subterranean parts as bulbs, rhizomes, corms, tubers, and stolons are produced in the following season.

Characteristics of Angiosperm:

• The sporophyte can be differentiated into stems, roots and leaves.
• At some point in Angiosperm’s life, all plants have flowers.
• The flowers are the plant’s reproductive organs, which provide them with a way to share genetic material.
• Angiosperms are vascular seed plants and the ovule is fertilized and develops into a seed within an enclosed hollow ovary.
• In Angiosperms, the ovary lies within the flower.
• Flower is the part of the angiosperms that comprises the male or female reproductive organs or ie. both the stamens (microsporophyll) and the carpels (megasporophyll) are arranged.
• Four microsporangia are present in each microsporophyll.
• The ovules are enclosed at the base of the megasporophyll in the ovary.
• Fruits are formed from the angiosperm plant’s maturing floral organs, and are thus typical of angiosperms.
• In contrast to non-vascular plants such as bryophytes, where each cells of the body is responsible for the functions essential for the support, nourishment and to expand plant body, angiosperms have developed specialized cells and tissues that perform these functions.
• In the xylem and companion cells in the phloem, the vascular system has real vessels.
• It comprises of extensive root systems that support the plant and is responsible for the absorption of water and minerals from the soil, stem that aids the growing plant, and leaves that are the major sites for the process of photosynthesis for majority of the angiosperms.
• The existence of localized plant growth regions called meristems and cambia, which extend the length and width of the plant body, respectively, is another important evolutionary development over the nonvascular and the more primitive vascular plants.
• These regions are the only places where mitotic cell division occurs in the plant body, except under some circumstances, although cell differentiation tends to occur throughout the life of the plant.
• Angiosperms are heterosporous, producing two kinds of spores, microspores (grains of pollen) and megaspores.
• Within the nucellus, a single functional megaspore is permanently retained.
• Transfer of the pollen grains from the anther to the stigma and reproduction takes place by pollination.
• They account for the transmission of genetic information from one flower to the other.
• The fertilization process in angiosperm is faster.
• Because of the smaller female reproductive parts, the seeds are also produced quickly.
• The pollen grains carrying the inherited information are produced by them.
• The developing seeds are enclosed by carpel, that may turn into a fruit.
• One of the main benefits of angiosperm is the production of endosperm.
• After fertilization, the endosperm is formed and is a source of food for seed and seedling growth.
• Angiosperms in a number of environments, including marine habitats, may survive.

Structure of Angiosperm:

• There are three parts to the basic angiosperm body: roots, stems and leaves.
• The vegetative (nonreproductive) plant body is constituted by these primary organs.
• The stem and its attached leaves, together, constitute the shoot.
• Jointly, the roots of an individual plant constitute the root system and the shoots the shoot system.

1. Root systems of angiosperm:

• The roots anchor a plant, accumulate minerals and water, and provide a food storage area.
• A primary root system and an adventitious root system are the two basic forms of root systems.
• Primary root system: The most popular form, the primary system, comprises of a taproot (primary root) that grows vertically downwards.
• Smaller lateral roots (secondary roots) that develop horizontally or diagonally are formed from the taproot.
• Such secondary roots also produce their own smaller lateral roots
• Thus, from a single prominent root, the taproot, many orders of roots of descending size are produced.
• Many dicotyledons generate taproots, such as the dandelion (Taraxacum officinale), for example.
• The taproot system is in some cases, modified into a fibrous or diffuse system in which the initial secondary roots are soon equal to or larger than the primary root.
• The outcome is many broad, positively geotropic roots that generate higher-order roots that can expand to the same size as well.
• Thus no well-defined single taproot exists in fibrous root systems.
• Fibrous root systems are typically shallower than taproot systems.
• Adventitious root system: The second root system type, the adventitious root system, varies from the primary variety in that the primary root is almost always short-lived and many roots that form from the stem substitutes them.
• There are adventitious roots in most monocotyledons; examples include orchids (Orchidaceae), bromeliads (Bromeliaceae), and many other tropical epiphytic plants.
• Grasses (Poaceae) and many other monocotyledons, with the development of adventitious roots, generate fibrous root systems.
• Adventitious roots, as in maize or some figs, are named prop roots when modified for aerial support.
• Wide woody prop roots grow from adventitious roots on horizontal branches in many tropical rainforest trees and provide additional anchorage and support.
• There are contractile adventitious roots in many bulbous plants that draw the bulb deeper into the soil as it grows.
• With specialized adventitious roots, climbing plants usually grasp their supports.
• Some lateral mangrove roots are specialized in saline mud flats as pneumatophores; pneumatophores are lateral roots that extend upwards (negative geotropism) for varying distances and act as the oxygen intake site for the submerged primary root system.
• For special functions, many primary root and adventitious root systems have been modified, the most common being the development of tuberous (fleshy) roots for food storage.
• For instance, carrots and beets are tuberous roots modified from taproots, and a tuberous root modified from an adventitious root is cassava (manioc).

2. Stem of angiosperm:

• The stem is an aerial axis of the plant bearing leaves and flowers.
•  It transports water and minerals from the roots and food from the site of synthesis to areas where it is to be used.
• Via a transition region called the hypocotyl, the main stem of a plant is continuous with the root system.
• The embryonic axis that bears the seedling leaves is the hypocotyl in the developing embryo.
• In a maturing stem, a node is called the area where a leaf connects to the stem, and an internode is called the region between successive nodes.
• At the nodes, stems bear leafy shoots (branches) that grow from buds.
• Lateral branches arise from buds located in the region between the leaf and the stem, either axillary or lateral, or from terminal buds at the end of the shoot.
• These buds have extended periods of dormancy in temperate-climate plants, while the duration of dormancy is either very short or non-existent in tropical plants.
• In order to understand the diversity of the shoot system in angiosperms, the precise positional relationship of the stem, leaf, and axillary bud is significant.
• Branching can be dichotomous or axillary in angiosperms.
• As a result of an equal division of a terminal bud (i.e. a bud developed at the apex of a stem), the branches develop in dichotomous branching .
• It is divided into two equal branches that are not derived from axillary buds, even if axillary buds are available elsewhere on the plant body.
• Some cacti, palms (Arecaceae), and bird-of-paradise plants are the few examples of dichotomous branching between angiosperms.
• The two angiosperm axillary branching modes are monopodial and sympodial.
• As the terminal bud begins to develop as a central leader shoot, monopodial branching occurs and the lateral buds remain subordinate, such as beech trees (Fagus, Fagaceae).
• Sympodial branching takes place when the terminal bud stops to develop (usually because a terminal flower has formed) and when an axillary bud or buds become new leader shoots, called renewal shoots, such as the Joshua tree (Yucca brevifolia, Asparagaceae).
• In general, plants with monopodial growth are pyramidal in shape, whereas those with sympodial growth mostly resemble a candelabra.
• By integrating monopodial and sympodial branching in one plant, several different tree configurations have developed.
• Example: In dogwoods (Cornus), the main axis is monopodial and the lateral branches are sympodial.
• By simply adjusting the length of the internodes, several different plant forms are formed.
• Excessive shortening of the internodes tends to result in rosette plants, such as lettuce (Lactuca sativa; Asteraceae), in which the leaves grow but the internodes between them do not elongate till the plant “bolts” while flowering.
• Excessive internode lengthening also leads to twining vines, as in the yam, (Dioscorea esculenta).

3. Leaves of angiosperm:

• A leaf base, two stipules, a petiole, and a blade compose the basic angiosperm leaf.
• The slightly enlarged area where the leaf attaches to the stem is the leaf base.
• When present, the paired stipules are situated on either side of the base of the leaf and may mimic scales, spines, glands, or structures that are leaflike.
• A stalk which connects the blade with the base of the leaf is the petiole.
• The main photosynthetic surface of the plant is the blade and it appears to be  green and flattened in a plane perpendicular to the stem.
• The leaf is considered simple when only a single blade is inserted directly onto the petiole.
• Simple leaves can be lobed along their margins in different ways.
• Simple leaves’ margins may be entire and smooth or they may be lobed in different ways.
• The rough teeth of dentate margins project at 90^o, while those of serrate margins point toward the leaf apex.
• Crenulate margins possess rounded teeth or scalloped margins.
• In one of two patterns, pinnate or palmate, the leaf margins of simple leaves can be lobed.
• The lamina i.e.the leaf blade is indented equally deep along each side of the midrib in the pinnately lobed margins (as in the white oak, Quercus alba; Fagaceae).
• The lamina is indented along many major veins in the palmately lobed margins(as in the red maple).
• A wide range of base and apex shapes are also found.
• A blade has two or more subunits called leaflets in compound leaves.
• The leaflets ramifies from the single point at the distal end of the petiole in palmately compound leaves.
• A row of leaflets are formed on the either side of the extension of the petiole termed as rachis in pinnately compound leaves.
• Some pinnately compound branch again forming a second set of pinnately compound leaflet.
• In bipinnately or tripinnately compound leaves, the high compoundness makes them appear to be shoot systems.
• However, they can still be differentiated since axillary buds are located at the angle between the stem and the petiole (axil) of pinnate or palmate compound leaves, but not in the leaflet axils.
• Alternate, opposite (paired), and whorled are the three patterns of leaf arrangement on stems in angiosperm.
• The leaves are present as single at each node in alternate-leaved plants, and are found along the stem alternately in an ascending spiral.
• The leaves are paired at a node and placed opposite to each other in opposite-leaved plants.
• When there are three or more evenly spaced leaves at a node, a plant has whorled leaves.

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General features of reproductive structures:

• In Angiosperms, there is a wide range of morphology and structure of the reproductive organs of the plant.
• Flowers being the reproductive tissues of the plant comprises of both the male and female reproductive organs in it.
• They may be found at the termination of short lateral branches or the main axis or both.
• Flowers can be found either individually as in daffodils or in clusters termed as inflorescence as in sunflower.
• The floral parts of the angiospermic plants give fruits.
•  A complete flower comprises of the four organs that are attached to the floral stalk via the receptacle.
• The four organs are sepals, petals, stamens and carpels which is located above the base of the receptacle.
• In case of dicots, the organs are usually grouped in the multiples of four and five, whereas, in case of monocots, the organs are grouped in multiples of three.
• The sepals are the outermost layer and are usually green in colour.
• The sepals encloses the flower bud and are collectively termed as calyx.
• The next layer of floral appendages inside the calyx are petals.
• Petals are generally bright in color and are collectively termed as the corolla.
• Jointly, the calyx and corolla form the perianth.
• Even if the sepals and petals protect the flower buds and attract the pollinators, they do not take part directly in sexual reproduction.
• Thus, they are termed as accessory parts.
• In case if the color and the appearance of sepals and petals are identical, then the perianth is said to be composed of tepals. For example: Easter Lily (Lilium longiflorum).
• The stamens are the spore producing structures (microsporophylls) and are located interior to the corolla.
• The stamens are collectively termed as androecium.
• The stamens comprise of a slender stalk (the filament) in most angiosperms, which contains the anther (and pollen sacs) inside which the pollen is produced.
• At the base of the stamens, tiny secretory structures called nectaries are usually found which supply food rewards for pollinators.
• The nectaries unify into a nectary or staminal disk in certain cases.
• In certain cases, as a whorl of stamens is reduced into a nectiferous disk, the staminal disc develops, and in some cases, the staminal disc is simply produced from the receptacle’s nectary-producing tissue.
• Megasporophylls are termed as carpels.
• Carpels enclose one or more ovules, each with an egg.
• The ovule matures into a seed after fertilization, and the carpel grows into a fruit.
• Carpels, and hence fruits, are special to angiosperms.
• A flower is termed to be complete when it possess all four organs, whereas, it is termed as incomplete when any one of it is missing.
• Both stamens and carpels are present in a bisexual (or “perfect”) flower.
• A unisexual (or “imperfect”) flower either lacks stamens and is termed as carpellate or lacks carpels and is termed as staminate.
• The term monoecious is given for the species where both the carpellate and staminate flowers are on the same plant.
• The term dioecious is given for the species where staminate flowers are on one plant and carpellate are present on the other.
• Floral organs are usually open or fused.
• The fusion of similar organs is termed as connation eg. the fused petals as in morning glory.
•  The fusion of different organs is termed as adnation eg. the fusion of stamens to petals in the mint family.
• The basic floral pattern comprises of the alternating whorls of the organs located concentrically from outside to inside as sepals, petals, stamens, and carpels, etc.

What are the reproductive structure of angiosperm?

1. Receptacle:

• The axis (stem) to which the floral organs are connected is termed as receptacle.
• The attachment of the floral organs are either in alternating successive whorls, found in majority of angiosperms or in a low continuous spiral form, as is common among primitive angiosperms.
• The stalk of a flower or of an inflorescence is termed as the peduncle.
• The peduncle is the internode between the receptacle and the bract when a flower is borne individually, where bract is the last leaf, usually modified and smaller in comparison to other leaves.
• In case where the flower are borne in an inflorescence, the internode between the receptacle of each flower and the bracteole is termed as a pedicel.
• Hence, in case of inflorescence, the pedicel is equivalent to peduncle whereas, bracteole is equivalent to bract.
• Usually the bract that subtends an inflorescence is bright in color like in the case of poinsettia (Euphorbia pulccherima) or provides the protection by woody boat shaped bracts in the cases of palms.
• In some angiosperms, as in strawberries the edible fleshy part is the receptacle.
• This, when eaten by birds and mammals helps in seed dispersal.
•  The fleshy portion of the edible fruit forms from the receptacle and peduncle in cacti (e.g., prickly pear), and several internodes below which develop up and cover the carpels; this is why there are axillary buds on the fruit surface in cacti (areoles) with spines.

2. Calyx:

• Owing to their usually green colour, the sepals (collectively called the calyx) most resemble leaves.
• Sepals remain separate (aposepalous or polysepalous) or slightly fused (synsepalous) from their base and along much of their length, forming a tube with terminal lobes or teeth.
• The number of calyx lobes equals the number of sepals that are fused (connate).
• The sepals enclose the unopened bud of the flower and protect it.
• In comparison to the more short-lived petals and stamens, the calyx is usually persistent and visible as the fruit matures (e.g., persimmon, Diospyros virginiana).
• When true petals are absent, sepals can be brightly colored and act as petals,  as in the virgin’s bower (Clematis) and the Bougainvillea.
• In this case, petaloid sepals vary from tepals because the first group of stamens is on the same radii as the sepals, indicating the absence of petals, which in the next floral whorl will usually be located on alternating radii.

3. Corolla:

• The petals constituting the corolla are usually brightly coloured or white and lure insects and birds for pollination.
• Typically, the number of petals is identical to the number of sepals.
• The petals describe floral symmetry.
• The flower has radial symmetry when the petals of the corolla are of the same shape and when they are equidistant from each other and the flower is named regular or actinomorphic.
• In regular flowers, the flower will be split into two identical halves by any line drawn across the middle.
• The flower has bilateral symmetry and is called irregular or zygomorphic if at least one petal of the corolla is different. Eg. violets.
• For all or part of their length, the corolla petals may be separate, or apopetalous, or marginally fused (fusion of similar floral parts is called connation), or sympetalous.
• They form a tubular corolla with terminal lobes when joined.
• In regular flowers e.g., blueberries or irregular flowers, eg. sage, a tubular corolla may be present.
• Stamens are generally associated with a tubular corolla.
• A marginally fused calyx is termed as synsepalous.
• A marginally fused corolla is termed as synpetalous.
• Synsepalous and synpetalous along with stamens fuse to form a cup like floral tube called a hypanthium that surrounds the carpel. For ex. Cherries.
• Fusion and reduction of flower parts are more frequent and have taken place in several unrelated lineages.
•  Several wind-pollinated angiosperms lack petals, nor have floral parts modified as petals; the amaranth family and the birch family are examples of wind-pollinated plants.
• Petals also hold nectaries that secrete compounds containing sugar, and petals often develop fragrances to attract pollinators; petals derive the scent of a rose.
• Petals also produce an extension of the tubular corolla containing nectar, called a spur.
• This may contain one petal, as in the larkspur, or all the petals, as in columbine, both being the members of the family Ranunculaceae.

4. Androecium:

• Stamens (microsporophylls) are pollen producing structures present in terminal saclike structures (microsporangia) termed as anthers.
• Sometimes the number of stamens comprised by the androecium is the same as the number of petals, but sometimes the stamens are more or less numerous than the petals.
• In a young stamen, there are usually two pairs of spore-containing sacs (microsporangia); the distinction between the adjacent microsporangia of a pair breaks down during maturation so that there are only two pollen-containing sacs (one in each anther lobe) at the time the pollen is released by the stamen.
• The less modified stamens, with the paired microsporangia located near the margins, are identical to leaves; an example is found in the magnolia family.
• The blade becomes modified into a slender stalk, the filament, with the microsporangia at or near the apex of the filament in more derived stamens.
• Generally, the filaments are attached with the corolla, but either isolate with the anthers, as in primroses (Primula; Primulaceae), or merged with each other to form a staminal tube enclosing the gynoecium, as in the mallow family.
• The staminal tube is fused with the lower half of the corolla tube in the thistle (Cirsium; Asteraceae) and in other members of the sunflower family.
• In stamen modification, there are many patterns.
• One or more of the stamens in many angiosperms are modified and lack functional anthers.
• The filament is extended in the most common modification to form a petal-like blade called a staminode.
• Apparent petals are of staminodial origin in some angiosperm families as seen in many members of Caryophyllaceae.
• Wild roses have only five petals and several stamens, but for the many apparent petals (but actually staminodes) and few usable stamens, cultivated roses have been selected.
• Stamens have been transformed into sterile nectaries involved in pollination in other situations.
• If flowers have a large number of stamens, then, as in the myrtle family, the stamens frequently occur in groups or clusters.

5. Gynoecium:

• Gynoecium is made up of carpels.
• Carpels are spirally arranged in more basal families (e.g., Magnoliaceae), and in more advanced families they appear to be arranged in a single whorl.
• The number of carpels ranges from one (e.g. Fabaceae family) to several (e.g. Raspberries).
• The ovary is at the base of a carpel, inside which one or more multicellular structures called ovules develop, each containing an egg.
• The pollen is received by the upper portion of the carpel is termed as the stigma.
• The ovary and the stigma are often connected by a slender stalk called the style.
• The carpels may be free (apocarpous) or fused (syncarpous), with the walls and cavities (locules) of the individual carpels still present.
• As in the wood sorrel (Oxalis), syncarpy may include only the ovaries, leaving the styles and stigmas free, or it may include both the ovaries and styles, keeping only the stigmas free, as in the waterleaf.
• The number of carpels in the syncarpous (or compound) ovary is generally identical to the number of locules.
• The location of the gynoecium on the floral axis with regard to the petals, sepals, and stamens also characterizes the flower.
• The perianth and stamens are connected to the receptacle below the gynoecium in hypogynous flowers; the ovary is superior to these organs, and the remaining floral organs originate from below the carpel’s point of origin.
• A hypanthium (a floral tube developed from the fusion of the stamens, petals, and sepals) is attached to the receptacle below the gynoecium in periginous flowers and surrounds the ovary; the ovary is superior, and the free parts of the petals, sepals, and stamens are attached to the hypanthium surface.
• The hypanthium is fused to the gynoecium in epigynous flowers, and the free parts of the sepals, petals, and stamens tend to be attached to the top of the gynoecium, as in the apple (Malus; Rosaceae); the ovary is inferior, and from the top of the ovary the petals, sepals, and stamens seem to emerge.

6. Fruit:                                                                                 

• Fertilization of an egg by a compatible pollen grain inside a carpel results in the development of seeds within the carpel.
• A ripened ovary (or compound ovary) and some other structure, usually the hypanthium, which ripens and forms a unit with it s termed as fruit .
• The formation of fruit without the fertilization of an egg and subsequent seed production is termed as parthenocarpy.
• As a vegetable is produced only from vegetative (non-reproductive) organs, this specifically distinguishes a fruit from a vegetable.
• Some examples of fruits are tomatoes, squashes, eggplants as they are derived from floral parts.
• From one single carpel or from a compound ovary, simple fruits grow.
• The aggregate fruits comprise of several single apocarpous gynoecium carpels. ex. Raspberries.
• In multiple fruits, gynoecia of more than one flower are found and represent a complete inflorescence as such fig and pineapple.
• In the development of the mature fruit, accessory fruits incorporate other flower parts; for instance, the hypanthium is used to form the pear (Pyrus; Rosaceae), and the receptacle becomes part of the prickly pear.
• Fruit shape, texture, and composition are variable (notably in simple fruits), but most of them fall into a few categories.
• There are three layers of the fruit wall or pericarp i.e. endocarp being the inner layer; the mesocarp being the middle layer; and the exocarp being the outer layer.
• These layers can be either fleshy or dry (sclerified) or either of the two variations, however they are either classified as one or the other.
• Berries, drupes, and pomes are the three primary types of fleshy fruits.
• Berries are simple fruits with several seeds, consisting of one carpel or a syncarpous ovary.
• Throughout, they are fleshy, but the texture of the exocarp varies: a smooth thin exocarp, like in tomatoes (a berry); a leathery exocarp, like in oranges (a hesperidium); and a very stiff exocarp, like in pumpkins.
• Typically, only one seed per carpel or locule is found in drupes, or stone fruits.
• Drupes are fleshy fruits consisting of an inner stony or woody endocarp adhering to the seed as in peaches and cherries.
• For each aggregate fruit unit of this sort, the word druplet is used. eg. raspberries.
• Pomes are fleshy fruits belonging to the rose family (Rosaceae) where the adnate hypanthium is fleshy.
• Simple dry fruits can be either dehiscent or indehiscent.
• If the pericarp splits open at maturity and releases the seeds, they are dehiscent or indehiscent while the pericarp stays intact when the fruit is shed from the plant.
• Follicles, legumes, and capsules are the three main forms of dehiscent fruits.
• From either single carpels or compound ovaries, indehiscent fruits are produced.
• The achene, the samara, and the caryopsis are single carpel forms.
• Nuts and schizocarps include forms derived from a compound ovary.
• An achene is a fruit in which the single seed in the cavity lies free, only connected by a single point.
• For instance, the strawberry is indeed an aggregate fruit, and each ‘seed’ is an achene.
• In the tree of heaven (Ailanthus altissima; Simaroubaceae) and ash, the samara that is a winged achene is found.
• The seed adheres to the fruit wall in the caryopsis, or grain.
• Among the cereal grasses, such as corn, the caryopsis is found.
• Nuts have a stony pericarp and as in oak acorns (Quercus; Fagaceae) and hazelnuts, typically only a single seed matures in each carpel.
• Schizocarps are fruits that divide each carpel of a compound ovary into two or more components, each with a single seed.
• In the carrot family, schizocarps are found.
• In maples, winged schizocarps are found.

7. Seed:

• The mature ovules are seeds.
• For the seedling, they provide the developing embryo and the nutritive tissue.
• Seeds are surrounded by one or two integuments that grow into a typically hard seed coat.
• They are found in a carpel’s ovary and are thus protected from components and predators.
• The ovule is connected by a short stalk called the funiculus to the ovary wall before maturity.
• The area of connection to the ovary wall is termed as the placenta.
• The placental arrangement (placentation) in the angiosperm compound ovary is distinguished by the presence or absence of a central column in the ovary and the location of attachment.
• The placentae are positioned on a central column in axil placentation; partitions form chambers (locules) from the central column to the ovary wall that separate the placentae and attached ovaries from each other.
• Free-central placentation is similar to axile placentation, except the column is not attached to the ovary wall by partitions, so no locules are created.
• The ovules are connected to the base of the ovary in basal placentation, and the placentae are placed directly on the ovary wall or on its extensions in parietal placentation.
• Mature seeds are covered in integuments that could become stiff and stony, or that may have an outer fleshy sarcotesta with an inner stony sclerotesta, typically brightly colored.
• Seed coats might also be winged or variably ornamented with prickles or sclerified hairs.
• There may be an extra covering in certain seeds, the aril, which is an outgrowth of the funiculus.
• The tomato becomes slippery because of the aril.

8. Inflorescence:

• The clusters of flowers on a branch or system of branches is termed as inflorescence.
• On the basis of timing of their flowering and their arrangement on the axis, they are usually categorized.
• In case of indeterminate inflorescence, the youngest flowers that are last to be opened are placed at the top of the inflorescence in elongated axes, however, they are arranged in the center in case of truncated axes.
• At any distance from the main stem, branching and the associated flowers develop.
• There are diverse kinds of indeterminate inflorescences. They are racemes, panicles, spikes, catkins, corymbs, and heads.
• Racemes: A raceme is a type of inflorescence where a flower develops at the axil of each leaf through the elongated, unbranched axis.
• A short stalk called a pedicel terminates each flower.
• There is indeterminate growth of the main axis; thus the growth does not stop at the beginning of flowering.
• Spike: A spike is a raceme other than that the flowers are connected directly to the axis at the axil of each leaf instead of being attached to a pedicel.
• Cattail(Typha) is the example of spike.
• A spadix is considered the fleshy spike characteristic of the Araceae, and a spathe is referred to as the underlying bract.
• Catkin: A spike in which all the flowers are of only one sex, either staminate or carpellate, is termed as catkin (or ament).
• The catkin is normally pendulous and when the inflorescence as a whole is shed, the petals and sepals are reduced to assist in wind pollination. Ex:Oaks
• First the lower flowers open, and a corymb’s axis begins to produce flowers. Ex: hawthorn
• Corymbs: Corymbs are found in the hawthorn (Crataegus; Rosaceae). The flowers emerge from a common point and tend to be at about the same level if the axis is short or stunted.
• An umbel, is actually a flattened raceme as the internodes of the axis, or peduncle (the point of origin of the leaves and flower axes), are shortened so that the pedicels are of the identical length (eg. the carrot family).
• Head: A head is a raceme where the peduncle is flattened and the flowers are directly attached to it. Ex. Aster family.
• This leads to grouping of small flowers that are arranged so as they seem as a single flower.
• The ray (external) flowers have a well-developed zygomorphic corolla in several members of the Asteraceae (e.g., sunflowers), and the disk (internal) flowers have a small actinomorphic corolla.
• Normally, the inner disk flowers are complete flowers, and usually, the ray flowers are sterile.
• The main axis is branched in the compound indeterminate inflorescences such that the various inflorescences form off the main axis.
• A panicle is a branched raceme where the branches are racemes themselves. Eg: yuccas.
• The shift to flattened axes (corymbs and umbels) from elongated axes (racemes and panicles) results in inflorescences in which the flowers are placed close together. ex. Wild carrot.
• With compound spikes, catkins, corymbs, and heads, this organization is the same.
• The shift to flattened axes (corymbs and umbels) from elongated axes (racemes and panicles) results in inflorescences in which the flowers are placed close together.
• This close association facilitates successful pollination, and an inflorescence that appears to be a single flower is created by the intense condensation of the inflorescences, as in the head. Ex: sunflowers
• The youngest flowers are at the bottom of an elongated axis or on the outside of a truncated axis in the determinate (cymose) inflorescences. Ex. In the cymose umbel of onions.
• These inflorescences are determinate because the entire apical meristem generates a flower at the time of flowering; therefore the entire axis stops to develop.
• Each unit of the cyme includes dichasium, which consists of a central flower and two lateral flowers.
• The branching is mainly sympodial, and it may be compound inflorescence. Ex: catchfly.
• There is a one-sided cyme called a helicoid cyme in several monocotyledons.
• The cymose inflorescence if arranged at the nodes in pair, in the manner of false whorl is termed as verticillaster.
• Lastly, there are mixed inflorescences as for example, the cymose clusters arranged in a racemose way (eg. lilac) or other type of combinations.

Reproduction in angiosperms:

source: Apbiology

General features:

• The immense arrangement of the floral parts of the angiosperms is for the sexual reproduction.
• The life cycle of angiosperms comprises of sporophytic and gametophytic phase.
• The cells of sporophyte body is diploid (2n), and the sporophyte is the body of the plant that we observe.
• When preparing for the reproduction, the sporophyte undergoes meiotic cell division and gives rise to the gametophyte.
• Gametophyte are the reproductive cells that are haploid(n) in nature.
• Pollen grain is a two-celled stage microgametophyte that germinates into a pollen tube and gives haploid sperms via division.
• The embryo sac which is an eight-celled stage gives rise to the eggs.
• Being vascular plants, angiosperms have life cycle in which sporophyte phase is dominant and gametophyte is recessive.
• The sporophyte is green and photosynthetic whereas gametophyte relies on the sporophyte for the nourishment.
• The micro-gametophyte is reduced to 3-celled stage and the mega-gametophyte is of 8-celled stage in case of angiosperms.
• Pollination acts as a driver for the sexual reproduction by bringing these gametophytes in close association and hence facilitates for the fertilization.
• Pollination is referred as a process in which the pollens generated by the anthers is received by the stigma of the ovary.
• Fertilization takes place by the fusion of sperms and eggs in order to produce a zygote, which ultimately forms the embryo.
• The ovule develops into a seed after the fertilization and the ovary develops into a fruit.

a. Anthers:

• Four areas of tissue capable of developing spores are exposed by a transverse segment of the anther.
• These tissues are comprised of microsporocytes.
• Microsporocytes are diploid cells that are go through meiosis to form a tetrad of haploid microspores.
• The microspores become pollen grains and may separate eventually.
• The layer of cells below the dermis of the anther wall (the endothecium) develops thickness in the cell walls during pollen growth.
• The cell layer develops into a layer of nutritional cells immediately within the endothecium (the tapetum) that either secrete their contents into the region around the microsporocytes or lose their inner cell walls, separate from each other, and become amoeboid among the microsporocytes.
• The pollen grain develop a thick wall of two layers i.e. intine and exine.
• The intine is the inner layer that comprises of basically of cellulose and pectin.
• The exine is the outer layer that is comprised of sporopollenin.
• Sporopollenin is the highly decay resistant chemical.
• The exine posses one to many pores through which pollen grain germinates whereas the thick area of the exine is highly shaped.
• To form a two-celled microgametophyte, each microspore (pollen grain) divides mitotically.
•  One cell is a tube cell (the cell that grows into a pollen tube), and the other is a generative cell, which, as a result of further mitotic division, will give rise to two sperm.
• Therefore, only three haploid cells, the tube cell and two sperm, form a mature microgametophyte.
• At the two-celled stage, most angiosperms shed pollen, however, in some advanced cases, it is shed at the mature three-celled stage.
• After the maturation of pollen grains, the anther wall breaks either longitudinally or by an apical pore.

b. Ovule:

• Ovule is a sac like structure that is enclosed by layers of cell.
• It is responsible for production of megaspores.
• In angiosperms, the nucellus is termed as the megasporangium.
• One or two integuments arise close to the base of the ovule primordium after the initiation of the carpel wall, expand in a rimlike manner, and enclose the nucellus, that leaves only a small opening called the micropyle at the top.
• The existence of two integuments in angiosperms is plesiomorphic (unspecialized) and one integument is apomorphic.
• The existence of two integuments in angiosperms is plesiomorphic (unspecialized) and one integument is apomorphic.
• Three of the four megaspores degenerate, and the one that remains enlarges.
• The resulting megagametophyte generates the female gametes.
• Free-nuclear mitotic divisions are involved in this development (called megagametogenesis).
• The cell wall remains intact until the megagametophyte, or embryo sac, is formed, while the nucleus divides.
• There are usually eight nuclei in the embryo sac.
• In gametophyte formation in gymnosperms, free-nuclear mitotic division is also found.
• To either end of the embryo sac, four nuclei migrate.
• Then, one nucleus from each group migrates to the embryo’s center; they become the polar nuclei.
• The two polar nuclei combine in the centre of the embryo sac to form a fusion nucleus.
• To form three antipodal cells, cell walls form around each of the chalazal nuclei.
• Enlargement of the embryo sac during development leads to the loss of much of the nucellus.
• In 70 percent of the angiosperms in which the life cycle has been charted, this series of megasporogenesis and megagametogenesis, called the Polygonum type, occurs.
• Differences present in the remaining 30 percent shows derivations from the Polygonum type of seed development.

Pollination:

• Efficient pollination involves the transition to a stigma of the same species of pollen from the anthers and subsequent germination and development of the pollen tube to the micropyle of the ovule.
• The transfer of pollen is carried out by wind, water, and animals, mainly insects and birds.
• Wind-pollinated flowers, covered with sticky trichomes and sometimes branched stigmas, pendulous catkin inflorescences, and thin, smooth pollen grains, typically have an inconspicuous reduced perianth, long slender filaments and styles.
• Wind pollination is derived from angiosperm and has evolved in many different groups independently.
• For example, in the Heliantheae and Anthemideae tribes, wind pollination accompanied by floral reduction has independently evolved within the aster family.
• In only a few aquatic plants, water pollination occurs and is extremely complex and derived.
• There is a wide variety of angiosperm animal pollinators and a wide range of flowers adaptations to attract such pollinators.
• Beetles pollinate some of the living non-specialized families of basal angiosperm.
• The beetles feed on pieces of the perianth and stamens.
• Bees are responsible for more flowers being pollinated than any other animal community.
• Typically, bees feed on nectar and occasionally on pollen.
• By visiting flowers of several species, they may be general pollinators, or they may have modified (i.e. elongated) their mouth parts to various flower depths and become skilled in pollinating only a single species.
• Bee pollinated flowers typically have a zygomorphic, or bilaterally symmetrical, lower lip corolla that provides the bee with a landing platform.
• Either at the base of the corolla tube or in extensions of the corolla base, nectar is commonly produced.
• In orchids (e.g., Ophrys speculum), a high degree of co-evolution is common where the flower not only appears to resemble the female wasp of a specific species, but also generates the pheromone released by the insect that attract males of the species.
• By pseudo-copulation with the orchid flower, the male wasp affects pollination.
• Flies, butterflies, moths and mosquitoes are other insect pollinators.
• Since they look and smell like rotting meat, many flowers pollinated by flies are called carrion flowers.
• Birds, bats, small marsupials, and small rodents are vertebrate pollinators.
• Some bird-pollinated flowers, especially those pollinated by hummingbirds, are bright red.
• As their food source, hummingbirds depend solely on nectar.
• Bird-pollinated flowers (e.g., fuchsia) contain abundant amounts of nectar but little to no odor because birds have a very poor sense of smell.
• They normally open only at night, when the bats are the most active, and sometimes hang on long stalks of inflorescence, providing easy access to the nectar and pollen.
• Small marsupials pollinate some eucalypts (Eucalyptus)
• Whatever the agent of dispersal, when a pollen grain lands on a receptive stigma, the first stage of pollination is successful.
• The stigma surface may be wet or dry and is mostly composed of specialized glandular tissue; secretory transmitting tissue lines the style.
• Their secretions create an aura that nourishes the pollen tube as the style elongates and evolves.
• If mitosis has not yet occurred in the pollen grain in the generative cell, it does so at this point.
• Many angiosperms have developed a chemical framework of self-incompatibility to prevent self-fertilization.
• Sporophytic self-incompatibility is the most common form, where secretions of the stigmatic tissue or the transmitting tissue prevent incompatible pollen from germinating or developing.
• A second form, gametophytic self-incompatibility, includes the inability to fuse and form a zygote of the gametes from the same parent plant or, if the zygote forms, then it does not grow.
• Finally, the pollen tube passes through the micropyle via an ovule and penetrates one of the sterile cells on either side of the egg.
• Immediately after pollination, these synergids begin to degenerate.

Fertilization and embryogenesis:

• The pollen tube releases the two sperm into the embryo sac after penetrating the degenerated synergids, where one fuses with the egg and forms a zygote, and the other fuses with the central cell’s two polar nuclei and forms a nucleus of triple fusion, or endosperm.
• This is called double fertilization since another fusion process (that of a sperm with the polar nuclei) that resembles fertilization accompanies the true fertilization (fusion of a sperm with an egg).
• There is now a total chromosome complement (i.e., diploid) in the zygote and three chromosomes in the endosperm nucleus.
• To form the endosperm of the seed, which is a food-storage tissue used by the developing embryo and the subsequent germinating seed, the endosperm nucleus divides mitotically.
• It has been shown that, while they still undergo double fertilization, some of the more basal angiosperms actually form diploid endosperm.
• On the basis of when the cell wall develops, the three key types of endosperm formation found in angiosperms, nuclear, cellular, and helobial, are categorized.
• In the formation of nuclear endosperm, repeated free-nuclear divisions occurs.
• If a cell wall formation takes place, it will form after free-nuclear division.
• Cell-wall formation is associated with nuclear divisions in cellular endosperm formation.
• A cell wall is laid down between the first two nuclei in the helobial endosperm formation, during which one half forms endosperm along the cellular pattern and the other half along the nuclear pattern.
• The endosperm degenerates in many plants, however, and food is retained by the embryo (e.g. peanut, Arachis hypogaea), the remaining nucellus (e.g. beet), or even the seed coat.
• The least specialized endosperm type with nuclear and helobial forms derived from it is the cellular endosperm.
• To form a multicellular, undifferentiated embryo, the zygote undergoes a series of mitotic divisions.
• A basal stalk or suspensor forms at the micropylar end, which disappears after a very short time and has no apparent angiosperm feature.
• The embryo proper is at the end of the chalazal (the area opposite the micropyle).
• Embryo differentiation, such as the growth of cells and organs with unique functions, involves the development of a primary root apical meristem (or radicle) adjacent to the suspensor from which the root grows and the development one cotyledon (in monocotyledons) or two cotyledons (in dicotyledons) at the opposite end of the suspensor.
• A shoot apical meristem is the site of stem differentiation and differentiates between the two cotyledons or next to the single cotyledon.
• The mature embryo is a miniature plant with one or two attached cotyledons, consisting of a short axis.
• The epicotyl that extends above the cotyledon(s) includes the apex of the shoot and the primordia of the leaf; the hypocotyl, the transition region between the shoot and the root; and the radicle.
• Three different generations of angiosperm seed growth, plus a new entity are the parent sporophyte, the gametophyte, the new sporophyte, and the new one, namely, the endosperm.

The post Reproduction in Angiosperm and Reproductive structures appeared first on Online Biology Notes.

• Plant hormones are also termed as phytohormones (named by Thieman), growth factors, growth regulators, growth substances etc.
• Phytohormone is an organic substance, naturally produced in higher plants that regulate plant physiological process such as affecting growth and other functions remote from its place of production and active in very minute amounts.
• They can be either natural or synthetic, stimulatory or inhibitory in nature.
• They act at a distance from the place where they are formed.
• Three types of phytohormones are mostly recognized. They are:
  • Auxin
  • Gibberellin
  • Cytokinin

1. Auxin:

• An auxin is an organic compound responsible for promoting the growth of plants along the longitudinal axis when applied in low concentrations to shoots of the plants.
• Auxin is specifically concerned with cell enlargement or the growth of the shoots.
• Auxin is identical to Indole 3-Acetic Acid (C₁₀H₉O₂N, IAA), i.e. natural true auxin.
• The precursor of Indole 3-Acetic Acid is tryptophan and zinc play a role in its biosynthesis.
• Auxin exhibits polar movement i.e.
• Basipetal movement (from apex to base) in case of shoots.
• Acropetal movement (from root tip to shoot) in case of roots.
• Bioassay test: Bioassay is termed as the functional test of substance in living plants.
• The common bioassay tests of auxin are Avena coleoptile test and root growth inhibition test.

What are the physiological roles of auxin in plants?

• Besides the cell enlargement and growth, auxin (both natural and synthetic) are responsible for various other growth processes. They are:
• Cell elongation:
  • The cell elongation occurs only in the presence of auxin and the rate of elongation is directly proportional to the amount of auxin supplied, given no other factors are limiting.
  • However, relatively high concentrations of auxin show inhibitory effects on this phase of growth.
  • Auxin promotes the elongation of roots at its low concentrations, the growth of roots is retarded at higher concentrations.
  • Flowers need higher concentration of auxin for their growth.
  • Auxin also induces the elongation of coleoptiles and stems by cell enlargement.
  • Auxins are responsible for the elongation of petiole, mid rib and major lateral veins of the leaves.
  •  Hence, adenine aids in enlargement in detached leaves of radish and pea. Similarly, coumarin has been shown to promote expansion of leaves in some plants.
• Cambial activity:
  • During the spring season, the trees manifest growth by developing buds that later on open and elongation of young stems take place.
  • Auxin activates this resumed growth by cambial cells
  • The growth moves basipetally in the stems from developing buds.
• Callus formation and galls:
  • The auxins activate cell division.
  • When 1% IAA in lanolin paste is applied to a de-bladed petiole of a bean plant, prolific division of parenchyma cells occurs.
  • A swelling or callus tissue is formed at the point of application of auxin.
  • The amount of callus tissue formed is directly proportional to the concentration of IAA applied.
• Apical dominance:
  • Apical dominance is the major function of auxin.
  • The growth of lateral buds is suppressed until apical bud is present in the plants.
  • This inhibitory effect of terminal bud upon the growth of lateral buds is termed as apical dominance.
  • Skoog and Thimann (1934) first reported the relation of apical dominance
    with the auxin supply.
  • When agar block containing auxin b or IAA was kept on the decapitated shoot of broad bean (Vicia faba), the lateral buds, as might be expected, resulted in the usual suppression of growth.
  • But when the same decapitated shoot was re-headed with an agar block containing no auxin, these lateral buds resumed growth.
  • When NAA was used as auxin in field-grown tobacco plants, similar results were obtained.
  • Evidence of apical dominance has been practically used in solving the potato storage problem.
  • Potatoes, stored for some time, sprout and become sweet in taste, causing the grower to lose financially as its consumers hate the sweet taste.
  • But by inhibiting the growth of buds or ‘eyes’ by spraying potatoes with auxins such as indole butyric acid ( IBA) and NAA, sprouting (or in other words, prolonging dormancy) can stop sprouting; the effect lasts for as long as 3 years.
• Rooting of stem cuttings (Formation of adventitious roots):
  • It is a common observation that when the lower end is dipped in an acceptable rooting medium, the appearance of buds on a cutting promotes the growth of roots.
  • In accelerating root formation, developing buds are efficient.
  • The initiation of roots on the cuttings are often favoured by young leaves.
  • These findings contributed to the idea that the auxins synthesized in the buds and young leaves favour the root formation and are later translocated to the basal part of the cut.
  • IAA, NAA, 2,4-D, naphthalene acetamide (NAd) etc are the auxins most widely used for this function.
  • Auxin-induced rooting is also of considerable horticultural benefit as it allows cuttings to propagate those plants.
• Delay (or inhibition) of abscission of leaves:
  • By adding auxins on  the surface of  the lamina or on the cut surface of a debladed petiole, abscission of  the leaves may  be delayed or hindered.
  • Laibach (1933), who demonstrated that the extract of orchid pollinia is capable of preventing leaf dropping, first noted the regulating actions of auxins on abscission.
  • Since then, sufficient work in this direction has been carried out.
  • The delaying effect of IAA on the abscission of different plant organs has been shown conclusively by Addicott and Lynch (1955).
  • As for the abscission process, it has been proposed that the basipetal migration of a hormone from the blade to the base of the petiole retards the leaf drop.
  • Leaf blade removal removes the hormone supply to the abscission zone and thus causes the drop of the leaf.
• Flowering:
  • Auxins play role in modifying flowering by following ways:
  • Producing early flowering
  • Inducing flowering
  • Preventing or delaying flowering
• Fruiting:
  • Auxins play significant role in fruiting by altering it in one of the following ways :
  • Fruit setting:
  • The changes in the ovary leading to the development of the fruit is termed as fruit set.
  • These changes are generally induced after pollination and fertilization.
  • The development of fruit without fertilization is termed as parthenocarpy.
  • It is a common characteristics in plants and hence occurs frequently.
  • The parthenocarpy can be induced artificially by the aid of auxin.
  • For example, Yasuda (1934) demonstrated it by application of pollen extracts to cucumber flowers.
  • It was also observed that ovaries of many plants (orange, lemon, grape, banana, tomato etc.) could be induced to develop into seedless fruits by application of IAA in lanolin paste to their stigmas.
  • The various other auxins used for parthenocarpy are IPA, IBA, α-NAA, phenoxyacetic acid (POA), α- naphthoxyacetic acid (NOA) etc.
  •  Fruit thinning:
  • The trees, in many cases, bear a large number of fruits.
  • It leads to the inability of the trees to grow an average number of new flower buds.
  • Therefore, such trees must grow fruit either in alternate years (alternate bearing) or if yearly, the number of fruits is significantly reduced.
  • Clearly, these trees need thinning.
  • For the first time, fruit thinning was achieved in apples when naphthalene acetic acid added to flowers failed to set the fruits and actually caused a decrease in the set of fruits.
  • It is interesting to note that the only effective auxin that induces fruit thinning seems to be naphthalene acetic acid.
  • However, a-2,4,5-trichlorophenoxyacetic acid for thinning of pears and p-chlorophenoxyacetic acid for thinning of grapes are other examples of auxins used for fruit thinning.
  • Control of premature fruit dropping:
  • The development of an abscission sheet causes the falling of unripe fruits in many fruit trees causes significant losses in yield to the gardeners.
  • In several cases, such as apples, the problem has now been successfully overcome by the application of auxins.
  •  Auxins prevent the formation of the abscission layer and thus check the drop of the fruits before harvesting.
  • With 2,4-D and 2,4,5-trichlorophenoxyacetic acid as auxins, regulation has also been induced in citrus fruits (like oranges and lemons).
  • Improving the quality of fruits:
  • The different processes such as colouring, softening, sweetening and ripening are all involved in improving the fruit ‘s quality.
  • In apples, where the use of 2,4, 5-trichlorophenoxyacetic acid has significantly increased red pigments, the auxin effects on fruit colouration are most noticeable.
  • 2,4-D accelerated the ripening process when added to bananas as the auxin stimulates the conversion of starch into sugars.
  • Sugar accumulation has been reported in sugarcane by injecting 2,4-D, IBA or maleic hydrazide.
• Increase in respiration:
  • Auxins enhances the respiration process. It was first identified by James Bonner in 1953.
  • A direct relation between growth due to auxin treatment and rate of respiration has been found i.e., greater the growth, higher is the respiration.
  • Auxins are used to control the growth of weeds in the crop fields.
  • 2,4-D is sprayed for the weeds in the crop fields that acts as weed killer.
  • Graminaceous weeds are destroyed by 2,4-dichloropropionic acid.
• Increased resistance to frost damage:
  • When parsnip is treated by 2,4,5-T, the tops resist damage by frost.
  • In apricot fruits, the application of 2,4,5-T before the onset of frost caused less damage than the untreated fruits.
• Great weapon of war:
  • When auxins are applied in higher concentrations on enemy crop fields by means of air, it causes devastation of land and form the basis for biological warfare.

2. Gibberellins:

• E. Kurosawa, first discovered gibberellin from a fungus called Gibberella fujikoroi in the year 1926.
• A gibberellin is abbreviated as GA, for gibberellic acid.
• Gibberellin may be referred as a compound which is active in gibberellin bioassays and possesses a gibbane ring skeleton.
• However, there are other compounds (like kaurene) that are active in some of the assays but lack a gibbane ring. Such compounds have been termed gibberellin-like rather than gibberellins.
• Brian isolated pure sample of a single gibberellin and termed as gibberellic acid.
• The structure for gibberellic acid was given by Cross et al in 1961.
• More than 100 types of gibberellin are known, among them GA₃ is most common.
• Gas are common in all groups of plants, however it acts as a flowering hormone in angiosperms only.
• All gibberellin possess gibbane ring. Gibbane ring consists of 4 isoprene units (hence, 2 terpenes, di-terpenes).
• The 5 carbon compound isopentenyl pyrophosphate is the precursor of gibberellin.
• Bioassay test: Gibberellins are synthesized via the mevalonic acid (MVA) pathway.
• The biosynthesis of GA3 from MVA takes place by 18 or more steps or intermediates and about 15 associated compounds.

What are major Physiological effects of Gibberellin in plants?

• Genetic dwarfism:
  • In some plants, the mutation of a single gene causes dwarfism.
  • Such individuals are termed as ‘single gene dwarfs’.
  • In these plants dwarfism is due to shortening of internodes rather than reduction in number of internodes.
  • The use of gibberellins on such dwarfs causes them to elongate to the point of being indistinguishable from common tall plants.
  • Hence, gibberellin A3 treatment  has been used to overcome genetic dwarfism successfully in many single gene dwarf mutants like Pisum sativum, Vicia faba and Phaseolus multiflorus.
  • Gibberellin also induces leaf expansion.
• Bolting and flowering:
  • Rosette plants are marked by the prolific growth of leaves and the delayed growth of internodes.
  • But there is striking elongation in the internode before the reproductive process, so that the plant reaches 5 to 6 times the initial height.
  • The treatment of these ‘rosette’ plants with gibberellins stimulates bolting (or shoot elongation) and flowering under conditions that would normally preserve the rosette shape.
  • It is also possible to distinguish shoot elongation from flowering by controlling the amount of gibberellin applied.
  • The plant can bolt but not flower with low gibberellin dosages.
  • GA₃ hastens the flowering and flower yield in many plants such as Coriandrum sativum (coriander).
  • Gibberellin controls flowering in long day plants.
• Light inhibited stem growth:
  • The dark-grown plants showed better stem growth in comparison to light grown plants.
  • This inhibitory effect of light on stem elongation could be reversed by the use of gibberellins in plants as such Pisum sativum.
  • This clearly indicates that the gibberellin is the limiting factor in stem elongation.
• Parthenocarpy:
  • Gibberellins induce parthenocarpy more efficiently than auxins.
  • It has been found in plants such as Cucumis sativa (cucumber), Zepyranthes sp., Solanum melongena (brinjal).
• Breaking dormancy of seeds:
  • In the light sensitive seeds (lettuce, tobacco), the germination is retarded in dark.
  • The application of GA₃ allows the germination of seeds in dark as well.
• Breaking dormancy of buds:
  • Because of very low temperature, the buds produced in winter stays dormant till the next spring in temperate areas.
  • Gibberellin treatment overcomes the dormancy in such cases and replaces the light requirement for breaking dormancy.
  • It breaks dormancy in potato tubers as well.
• Role in abscission:
  • The abscission has been enhanced in explants of bean and Coleus by the GA₃ treatments.
• Stimulation of enzyme activity in cereal endosperm:
  • It was demonstrated that the exogenous application of gibberellins stimulated amylase activity in isolated barley endosperm.
  • It was also found that the treatment of isolated aleurone layer of endosperm with GA could release enzymes, amylase and proteinase.
• Sex expression:
  • Gibberellins show the capability to alter the sex of the flowers.
  • It promotes the production of male flowers in cucurbitis, Cannabis etc.
  • Also, the antheridia have been induced to form in many fern gametophyte
•  Juvenility:
  • Most of the plants manifests two different stages of growth i.e. a juvenile stage and an adult stage.
  • The application of gibberellin helps to determine if a specific part of plant is juvenile or not.

3. Cytokinins:

• Cytokinins are alos named as kinetins because of their absolute power to enhance cell division in the presence of an auxin.
• First naturally occurring cytokinin was recognized from young maize grain by Letham and termed as zeatin.
• Fox (1969) has defined cytokinins as chemicals composed of one hydrophilic adenine group of high specificity and one lipophilic group without specificity.
• Chemically, kinetin (C10H9ON5) is 6-furfurylaminopurine.
• Cytokinins occur in higher plants, diatoms, red and brown algae, mosses.
• These occur widely in embryo sac, roots during seedling stage, flowers, developing fruits, cambial tissue and endosperm.
• The richest source of kinins are fruits and endosperm.
• Bioassay test: Callus pith cell division, chlorophyll retention test, soybean and radish cotyledon cell division are the main bioassay tests.

What are Physiological roles of cytokinin in plants?:

• Cell division:
  • In addition to auxins, the kinins are required in right ratio of concentrations for the enormous growth response .
  • When mixture of auxin and cytokinin is added to unspecialized  cells, their differentiation begins.
  •  A high cytokinin to auxin ratio results the formation of shoots, buds and leaves while a low cytokinin to auxin ratio causes root formation.
  • This invitro culture methods allows the rapid production of large number of plants in a small space.
• Cell elongation:
  • Kinetin also enhances cell elongation.
  • It has been demonstrated in tobacco pith cultures, tobacco roots and bean leaf tissues.
• Root growth:
  • Kinetin is responsible for both the stimulation as well as inhibition of root development.
  • When kinetin was applied along with IAA, the root initiation and development in stem callus cultures was stimulated.
  • In lupin seedlings, Kinetins induced increase in dry weight and elongation of the roots.
• Shoot growth:
  • When the balance of IAA and kinetin is maintained, the callus tissue of tobacco can be kept in an undifferentiated state for a long time.
  • When the amount of kinetin is increased, the development of leafy shoots begins.
• Organogenesis:
  • Organogenesis is resulted by cytokinins in several tissue cultures.
  • By changing the relative concentrations of kinetins and auxins, the tobacco pith callus can be directed to develop either buds or roots.
  • High kinetin and low auxin contents causes the production of buds.
  • The roots appear in pith in reverse condition, i.e. high auxin and low kinetin contents.
  • In leaf segments of various plants such as Saintpaulia ionantha, Bryophyllum sp and Begonia sp., the kinins stimulate the production of buds.
  • In addition to the root and shoot differentiation, the cytokinins also bring about other morphogenetic responses.
  • These are :
    (a) maturation of proplastids into plastids
    (b) differentiation of tracheids
    (c) induction of parthenocarpy
    (d) induction of flowering
• Counteraction of apical dominance:
  • Cytokinins are powerful promoters of lateral bud growth.
  • When the culture medium consists of IAA, the growth of lateral buds is inhibited, but the addition of kinetin along with IAA stimulates the growth of lateral buds.
• Breaking dormancy of seeds:
  • Cytokinins show effective role in breaking seed dormancy in lettuce, tobacco, white clover and carpet grass.
  • In such cases, the site of cytokinin action is cotyledon.
  • The seeds of parasites such as Striga asiatica need the host plant for germination. But when treated with kinetin, the seeds germinate even in the absence of their host.
• Delay of senescence (Richmond-Lang effect):
  • The ageing of leaves along with the loss of chlorophyll and the breakdown of proteins is termed as senescence.
  • Richmond and Lang demonstrated that the senescence in the detached leaves of Xanthium could be postponed for several days by kinetin treatment.
  • This effect of kinetin in retarding senescence is termed as Richmond-Lang effect.
• Role in abscission:
  • Depending on the site of application, cytokinins can either accelerate or retard the process of abscission in leaf petioles.
  • It is the common property of cytokinin.
•  Effects on cotyledons:
  • Cytokinins enhances cellular division and expansion in cotyledons.
  • Cytokinins increase the concentration of sugars in cells resulting in endosmosis that causes the expansion of cytokinin treated cells in cotyledons.

Phytohormones: Types and physiological effects in plant growth and development

The post Phytohormones: Types and physiological effects in plant growth and development appeared first on Online Biology Notes.

• The loss of water in the form of vapor from the living tissues of aerial parts of plant such as leaf, stem, leaves etc. is termed as transpiration.
• The plants uptake abundant quantity of water from the soil through their root hairs. Some portion of the water is utilized in the metabolic activities of the plant whereas rest of them are evaporated from the stem and the leaves.
• Transpiration takes place through stomata, lenticels or cuticle.
• Transpiration is a metabolic process regulated by protoplasm and may be decreased or increased where needed by the nature.
• It differs from evaporation in fact that transpiration being a physiological process while evaporation is a physical process.
• The rate of transpiration is measured by potometer.

Types of transpiration in plants:

• On the basis of site of transpiration, there are three types of transpiration. They are:
  1. Stomatal transpiration: It occurs through the stomata situated on the leaves and sometimes on the green stems. It is the most important one. Almost 90-97% of the total transpiration occurs through the stomata.
  2. Lenticular transpiration: It occurs through the lenticels found on the stem. The stomata remain closed during night and the plant transpire through lenticels and cuticle.
  3. Cuticular transpiration: It takes place through the cuticle found on the surface of the stem and leaves.

Factors affecting transpiration:

External factors:

1. Light:
   • Transpiration is indirectly affected by light.
   • The rate of transpiration is always greater in light than in darkness.
   • In presence of light, the stomata remain open and hence transpiration takes place whereas in night time, the stomata are closed and transpiration is checked.
   • Only lenticular and cuticular transpiration occurs during night.
   • The efficient transpiration is induced by blue light followed by red light.
   • In presence of green light, UV rays, and infrared light, stomata never opens.
2. Atmospheric temperature:
   • The rate of transpiration is directly proportional to atmospheric temperature.
   • When the temperature is high, transpiration from the leaf surface occurs rapidly.
3. Atmospheric moisture:
   • Dry and low humidity enhances the rate of transpiration.
   • The rate of transpiration decreases in saturated atmosphere.
4. Wind velocity:
   • The rate of transpiration increases in windy condition, however, when the atmosphere is calm, transpiration rate decreases because of moist air in proximity of transpiring plants.
5. Solar radiation:
   • Solar radiation is deeply related to transpiration.
   • Due to solar radiation, the temperature of the leaves rises, thus increasing the rate of transpiration.
6. Soil environment:
   • It indirectly affects transpiration.
   • If the absorption of water from soil is abundant, the transpiration occurs to a larger extent and vice-versa.

Internal factors:

1. The root system:
   • If the roots are deeply penetrated to the soil in moist layers, there is abundant absorption of water and at same time, transpiration increases too.
   • However, if the roots are limited to the upper layers of the soil, there is less absorption of water and less transpiration.
2. The stem:
   • The rate of transpiration depends on diameter of xylem vessels present in the stem.
   • If the xylem vessels are wider, there is increase in transpiration and vice-versa.
3. Leaf structure:
   • Several adaptations in plants are present in order to check transpiration.
   • In case of xerophytes the area of leaves is decreased because of many divisions of leaves.
   • Sometimes, the leaves are completely absent and the stems are flat, angular or rounded. These are called phylloclades. Ex- Ruscus, Opuntia.
   • Transpiration is less from the surface of phylloclades.
   • The leaves of some plants bear special depressions where stomata are situated and are surrounded by hairs, this aids in checking transpiration. Ex. Nerium, Banksia, etc.

Significance of transpiration:

• As transpiration helps in the movement of xylem sap, it increases the absorption of mineral nutrients by the roots from the soil.
• It causes cooling effect on leaf and plant surface.
• It produces suction pressure for absorption, ascent of sap, mineral translocation and distribution if minerals.
• Transpiration decreases heating of leaves by solar radiations.
• It maintains turgidity as well as aids in hydrological cycle.

disadvantages of transpiration:

• The energy used during absorption is wasted.
• Unwanted loss of water.
• Excess transpiration causes wilting that is harmful for plants.
• It increases acidity, alkalinity or aridity of soil.

Mechanism of stomatal transpiration:

 Structure of stomata:

• The stomata (stoma, singular) are microscopic apertures commonly found on the epidermis of leaves, green fruits and herbaceous stems.
• Stomata are never present in roots.
• It is biconvex elliptical in structure.
• The two kidney-shaped special epidermal cells termed as guard cells surrounds each stoma.
• The guard cells are filled with thin layer of cytoplasm and central large vacuole.
• The cell wall of guard cells surrounding the stomatal pores is thicker and inelastic because of the formation of secondary layer of cellulose, while rest cell wall is thin and elastic.
• The epidermal cells that surrounds the guard cells of the stoma are termed as accessory or subsidiary cells.
• The guard cells are always living and consists of small chloroplasts unlike other epidermal cells.
• In case of dicotyledonous leaves, the stomata are found scattered whereas in case of monocotyledonous leaves, the stomata are arranged in parallel rows.
• The stomata may be found on both the surface of the leaf, but their number is always greater on the lower surface.

Shape, size and number of stomata:

• The shape of guard cells in case of dicots is reniform or kidney shaped whereas in case of monocots, it is dumb-bell shaped.
• The size of stoma varies from species to species and measures 3- 12 μ.
• The number of stomata can vary from thousands to lacs per square centimetre on the surface of the leaf.

Mechanisms of opening and closing of stomata:

• In normal condition, the stomata remain closed in the absence of light.
• In the day time or in the presence of light, stomata are always open.
• Under each stoma, a respiratory cavity is present.
• The mechanism of the closing and opening of the stomata relies upon the presence of sugar and starch in the guard cells.

• Photosynthetic guard cells:
  • According to Von Mohl and Schwendener, sugar is produced by the chloroplasts of the guard cells through photosynthesis.
  • The sugar is soluble and hence increases the concentration of sap of guard cells.
  • The increase of osmotic pressure of guard cells leads to endosmosis of water from neighbouring cells into guard cells and they become turgid.
  • This results in opening of stomata.
• Starch-sugar inter conversion hypothesis:
  • This hypothesis states that the opening and closing of stomata is controlled by phosphorylase enzyme.
  • During daytime, the starch converts into glucose (sugar) by the activity of phosphorylase enzyme.
  • The increasing concentration of sugar in the guard cells causes endosmosis from neighboring cells.
  • Hence, the guard cells become turgid and stomata opens
  • The sugar present in guard cell converts into the starch in the absence of light or during night.
  • The starch is insoluble, and hence the cell sap of the guard cell remains of much lower concentration in comparison to neighboring cells.
  •  Exosmosis from the guard cells takes place by making them flaccid and the stomata is closed.
  • The starch-sugar inter-conversion depends upon the acidity (pH) and alkalinity of the cell sap of guard cells.
  • During night, photosynthesis is absent thus the carbon dioxide gets accumulated in the guard cells.
  • This converts the cell sap in to weak acidic starch.
  • The carbon dioxide is utilized in the process of photosynthesis during daytime and the cell sap becomes alkaline and the starch converts in to sugar.
• Concentrations of CO₂ hypothesis:
  • This hypothesis for opening and closing of stomata was proposed by Bonner and Galston.
  • It relies upon the concentration of the carbon dioxide (CO₂) present in the stomatal chamber.
  • It is independent of the presence or absence of light.
  • Normally 0.03% of carbon dioxide is present in the atmosphere.
  • When the density of the CO₂ in the sub stomatal chamber also becomes 0.03%, then the guard cells become flaccid and the stomata becomes closed.
  • As the density of CO₂ decreases gradually, the stoma starts to open and it opens gradually lengthwise until the density of CO2 becomes 0.01%.
  • Now the stomata are completely open and they are not open further beyond this density.
  • The photosynthesis occurs in day time and much of the carbon dioxide is being utilized in the process, the density becomes lesser than 0.03% and the stomata stays open during day time.
  • During night or in the darkness, photosynthesis is absent, the density of carbon dioxide remains 0.03%.
  • The guard cell remains flaccid and the stomata remains closed.
• Active potassium (K⁺) theory:
  • This theory is also termed as hormonal regulation theory or malate switch theory or potassium malate theory.
  • This theory was proposed by Levitt in 1974.
  • The role of potassium (K⁺) in stomatal opening is now most accepted world-wide.
  • In 1967, Fujino, for the first time observed that opening of stomata takes place due to the influx of K⁺ ions concentration.
  • The osmotic concentration of guard cells is increased by the influx of K⁺ and causes stomatal opening.
  • The uptake of potassium K⁺ controls the gradient in the water potential.
  • This in turn triggers endosmosis into the guard cells increasing the turgor pressure.
  • ATP aids in entry of K+ ions into the guard cells.
  • Levitt (1974) observed that proton (H+) uptake by the guard cell’s chloroplasts occurs with the help of ATP.
  • This leads to rise of pH in guard cells.
  • Increase in pH converts starch into organic acid, such as malic acid.
  • Malic acid again dissociates to form H+ and malate anion.
  • The absorption of potassium K+ ions is balanced by one of the following:
    • Uptake of Cl-
    • Transport of H⁺ ions from organic acids, such as malic acid
    • By negative charges of organic acids when they lose H⁺ ions
  • The accumulation of large concentration of K+ ions in guard cells is ionically balanced by the uptake of negatively charged ions, i.e., chloride and malate.
  • The hydrolysis of starch causes the accumulation of high amount of malate in guard cells of open stomata.
  • A passive or highly catalyzed excretion of K+ and Cl from the guard cells to the epidermal tissue results in stomatal closure in general and subsidiary cells in particular.
  • It is considered that subsidiary cells have an active re-absorption mechanism of K⁺.

Factors Affecting Stomatal Movement:

• The most likely factors that affect opening and closing of stomata include:
• i) Light:
  • It has intense controlling affect on stomatal movements.
  • Generally, stomata open in light and close in darkness.
  • Exception, the stomata of plants showing CAM (Crassulacean Acid Metabolism) such as pineapple agave, aloe, opens during night and closes in day time. Even moonlight is enough for the opening of stomata.
  • The concentration of light needed to achieve optimum stomatal opening differs from species to species. For instance, some plants, such as tobacco need low light intensities, while others may need full sunlight.
  • However, light intensity required for stomatal opening is very low than the intensity required for photosynthesis.
  •  The duration during which stomata remain open in daylight and close at night changes from species to species of plants.
  • Different wavelengths of light affect stomatal movement on different ways.
• ii) Temperature:
  • Generally, the rise in temperature causes the stomatal opening unless water is a limiting factor.
  • Even under continuous light at 0^oC, stomata remain closed in some plant species.
  • For example, in Camellia (tea Plant), stomata remain closed at very low temperature (below 0^o C) even in strong light.
  • There is decline in stomatal opening at temperatures higher than 30^oC in some species.
• iii) Water availability:
  • In the condition, where water availability is less and the rate of transpiration is high, plants goes through water stress.
  • Water stress is also termed as water deficit or moisture deficit.
  • Such plants start to show signs of wilting and are referred as water-stressed plants.
  • Under such conditions, most of the mesophytes, close their stomata tightly and completely to protect them from the damage that may result due to extreme water shortage.
  • The stomata reopen only when water potential of these plants is restored.
  • This type of control of stomatal movement by water is called hydro-passive control.
  • In the guard cells of several water stressed plants, accumulation of phytohormone abscisic acid (ABA) is now well established.
  • The closing of stomata of such plants is caused by ABA.
  • When water potential of water-stressed plant is restored, the stomata reopen and ABA gradually disappears from the guard cells.
  • This type of control of stomata by water, communicated through ABA, is termed as hydro-active control.
  • ABA, when applied externally to leaves of normal plants also includes closure of stomata.
• iv) Carbon-dioxide (CO₂) concentration:
  • CO₂ concentration has noticeable effect on opening and closing of stomata.
  • Opening of stomata is favored by reduced CO₂ concentrations while an increase in CO2 concentration causes stomatal closing. 
  • This occurs even in presence of the light.
  • In specific species of plants, stomata also close merely by breathing near leaves.
  • Stomata, that are forced to close by raised CO2 concentration, do not reopen easily simply by draining the leaf with free and dark CO2 air.
  • However, such stomata soon open during subsequent light exposure.
  • This occurs because, during light exposure, CO2 stored within the leaf is absorbed in photosynthesis.
  • This suggests that the internal leaf CO2 concentration is responsible for stomatal opening rather than ambient carbon dioxide.
  • The cuticle over the guard cells and epidermal cells, however, is rather impermeable to CO2 and ensures that stomata respond to the CO2 present in the leaf rather than the external atmosphere.
  • Some endogenous factors also affect stomatal movement such as K+, Cl+ and H- ions and organic acids.

Transpiration in plants: types, mechanism, affecting factors and significance

The post Transpiration in plants: types, mechanism, affecting factors and significance appeared first on Online Biology Notes.

• The functional study of live processes is termed as physiology.
• Plant physiology deals with water relations (such as diffusion, osmosis, absorption, transpiration, and ascent of sap), photosynthesis, respiration, photorespiration, growth hormones, movements and locomotion, vernalization and seed germination.
• Plant absorbs water and soluble mineral salts from soil by means of root system.
• The unicellular hairs present on the roots facilitate the absorption.
• Plant cells comprises of cell wall and protoplast.
• The term protoplast is usually used to refer collectively to the plasma membrane and protoplasm.
• Normally, the plant cell comprises of three components:
  • Vacuole
  • Protoplasm
  • Cell wall
• These compartments are separated from each other by plasma membranes i.e., the tonoplast is found between the vacuole and the protoplasm, whereas plasma lemma is found between the protoplasm and the cell wall.
• The protoplasm of one cell is connected to the other cell by plasmodesmata.
• The plasma membrane is selectively permeable in nature i.e. it permits some materials to pass through and not others.
• The water is the most essential factor for the vital functioning of the plants.
• The plants fail to survive in its absence.
• The plant cells become turgid by water and it gives temporary mechanical support to the young plants.
• The small portion of water absorbed by water is retained in plant cells whereas most portion of water goes out of the surface of plants by a vital process termed as transpiration.

Availability of water in the soil:

• There are about three forms of water in the soil.
  1. Free water: Due to gravitation, some amount of water just after the rains along with some minerals goes in the lower strata of the earth. This is termed as gravitational or free water and cannot be utilized by the plants.
  2. Hygroscopic water: Besides free water, every particle of soil captures some imbibed water in it. The imbibition force is greater and hence the plants cannot absorb from the soil. This is termed as hygroscopic water which also cannot be absorbed from soil by the plants.
  3. Capillary water: Each soil particle is surrounded by a loose film of water, the capillary force attracted this film to the soil and hence this water film is termed as capillary water. Only capillary water can be absorbed by the plants. In this water, mineral salts are also found and are hence absorbed by plants along with water. If soil lacks the capillary water, the plants starts to wither and finally dies.

Water potential:

• The potential energy possessed by water is termed as water potential.
• The tendency of water to leave the system is referred to as water potential.
• The term is often used to explain the direction in which the flow of water will take place from one cell to another, or from one part of the plant to another.
• Water always flows from a region of higher water potential to the region of lower water potential.
• Osmotic movement of water involves the particular work done.
• The major driving factor behind this movement is the difference between free energies of water on two sides of the selectively permeable membrane.
• The free energy molecule for water is termed as water potential (ψ_w)
• Water potential is measured in terms of pressure.
• The unit is Pascal, Pa.
• The water potential of pure water is zero at atmospheric pressure.
• Hence, all the solutions at atmospheric pressure have lower water potential than water, that means they have a negative value.
• Water potential ψ_w is expressed as the difference between the potential of a solution in a given state and the potential of the same solution in a standard state.
• The water potential is lowered by the addition of solutes.
• Since the water potential of pure water is 0, all other water potential values will be negative.
• Hence, the movement of water occurs in osmotic or other systems from a region of higher water potential (i.e. less negative) to a region of lower water potential (i.e. more negative).
• Water potential of any solution is affected by three factors:
  • Concentration
  • Pressure
  • Gravity
  • This can be represented by the equation as:
  • ψ_w = ψ_s + ψ_p + ψ_g
  • ψ_s = effect of solutes (i.e. solute potential or osmotic potential). Solutes in a cell decrease the free energy of water, or the water potential.
  • ψ_p = effect of pressure (i.e. pressure potential or hydrostatic pressure). The positive hydrostatic pressure is termed as turgor pressure.
  • ψ_g  = effect of gravity (i.e. gravity potential). This term refers to the effect of gravity on water potential. It relies on the height of the water. If the vertical height is less than five meters, the ψ_g is negligible.
• In case of plant cell, only ψ_s and ψ_p are essential and considered i.e., ψ_w = ψ_s + ψ_p
• According to the equation, when water moves into the cell from outside, the hydrostatic pressure, i.e. pressure potential (ψ_p) increases, that results in an increased water potential (ψ_w) of the cell, and the difference between the inside and outside ψ_w is decreased.
• On the other hand, the concentration of solute when increases in the cell, the solute potential (ψ_s) is lowered, and thus, water potential (ψ_w) is reduced.
• Thus, due to water potential gradient the water from outsideflows inside the cell.
• The water moves out of the cell if a pressure is applied on the cell.
• The external pressure raises the water potential (ψ_w ) of the cell, and hence the difference in water potential inside and outside will be such that water will expel out.
• Hence, the two basic factors that affect the water potential are amount of solute and external pressure.

1. Diffusion:

• The diffusion is a passive movement of individual molecules in all directions from a region of higher concentration to a region of lower concentration.
• The flow of molecules is directly proportional to the difference in concentration.
• Diffusion is more faster in gases than in liquids.
• In case of diffusion, the movement is random and is independent of each other.
• It occurs along concentration gradient and kinetic energy and converts solvent into solution.
• The imaginary potential of solute particles to diffuse from its higher concentration to lower concentration is termed as diffusion pressure (DP).
• Its value is always greater for a pure solvent than its solution.
•  If sugar solution is made in water, then diffusion pressure is lower than that of water.
• The rate of diffusion is directly proportional to temperature, kinetic energy, and diffusion pressure gradient.
• The rate of diffusion in inversely proportional to density of medium, humidity, size of solute, molecular weight of solute and distance between diffusing particles.
• Examples of diffusion are loss of water vapour from leaves to the atmosphere, and supply of carbon dioxide from atmosphere to the leaves for photosynthesis.
• Every liquid has a fixed diffusion pressure. A pure solvent has maximum diffusion pressure.
• When some amount of solute is added to it, it’s diffusion pressure decreases. This deficit in diffusion pressure of the solution due to the addition of solute is termed as Diffusion pressure deficit (DPD).

2. Osmosis:

• It is a special type of diffusion.
• It involves the movement of water from its low concentrations to high concentrations through semi permeable membrane.
• When concentrated and dilute solutions are separated by semi permeable membrane, the solvent moves from a concentrated solution to dilute solution. This process is termed as osmosis.
• Osmosis involves movement of solvent from higher water potential to lower or from high free energy to lower free energy.
• Osmosis does not take place in isotonic solutions.
• Osmosis is feasible only in living cells.
• Osmosis continues till the equilibrium occurs between hydrostatic pressure and osmotic pressure.
• Osmosis can be either endosmosis or exo-osmosis.
• During endosmosis, the movement of solvent takes place into the cells from the surrounding, and the cell becomes turgid.
• During exo-osmosis, the movement of solvent takes place towards the surrounding from the cell, and the cell becomes flaccid.
• Osmotic pressure (OP) or Solute potential (ψ_s):
  • The pressure developed in a solution when it is separated from it’s pure solvent by means of semipermeable membrane is termed as osmotic pressure (OP).
  • The OP of pure solvent is regarded as 0, so OP for solution is always positive.
  • Osmotic pressure is directly proportional to diffusion pressure deficit and concentration of solution.
• Turgor pressure (ψ_p):
  • The hydrostatic pressure that is generated when solvent particles enter the cell and the cell membrane pushes the cell wall is termed as turgor pressure (TP).
  • Turgor pressure is only applicable for osmotic solution.
  • Flaccid cell has zero turgor pressure, turgid cell has maximum OP, TP near to minimum.
  • In case of plasmolyzed cells, negative TP is believed to be present.
  • Turgor pressure maintains turgidity and growth of cell.
  • It provides necessary energy to plumule to come out and aids in penetration of radicle into soil during germination of seeds.
• Wall pressure (ψ_w ):
  • When the cell wall becomes slightly rigid, it exerts an equal and opposite pressure in a turgid cell and is termed as wall pressure.
  • In case of turgid cells, it is equal to turgor pressure.
• Diffusion pressure deficit (DPD):
  • The difference between the diffusion pressure of the pure solvent and its solution is termed as diffusion pressure deficit.
  • DPD is the osmotic parameter that is reason for the entry of water into the plant cell.
  • The direction of osmosis is determined by DPD.
  • It is also termed as suction pressure.
  • The difference between OP and TP is known as DPD.
  • DPD = OP-TP
  • Normally, OP is greater than the turgor pressure.

Experiments demonstrating osmosis

• Egg membrane experiment:
  • An egg is taken and a hole is formed on the pointed side of the egg.
  • Then all the white and yellow part of the egg is taken out through the hole.
  • Now, the empty egg is placed in the dilute solution of hydrochloric acid (HCl), such that the hard shell of the egg made up of calcium carbonate is dissolved in it.
  • The membrane of the egg, present just beneath the cell remains intact.
  • Now, this egg membrane is thoroughly washed with tap water, then a glass tube is inserted in it and tied tightly with a piece of thread, so that the end of the glass tube remains inside the egg membrane.
  • Then, the concentrated solution of sugar is poured in it with the help of glass tube by thistle funnel.
  • The sugar solution is filled up in such a quantity, so that after completely filling the egg membrane sac, it also reaches to a small height in the glass tube.
  • The sugar solution filled egg membrane sac is then hanged in a beaker that is filled up with water, so that the level of the inner and outer liquid is same.
  • This apparatus is left for some time in the same position.
  • After some moments it is to be noted that the level of the liquid inside the glass tube has risen up enough from the level of the water of the beaker.
  • Water will continue to move across the membrane, resulting in the rise of solution in the funnel until an equilibrium is reached. Thus, osmosis is demonstrated.
  • From upper part of funnel, pressure can be applied to the solution to prevent movement of water into it through egg membrane.
  • The pressure required to halt the movement of water totally is termed as osmotic pressure.
  • This can be termed as osmotic potential or solute potential.
  • Osmotic pressure and osmotic potential are numerically equal but osmotic potential has a negative sign.
  • If one reverses the experiment by filling up the water in the egg membrane sac and the sugar solution in the beaker then the water moves from inner side to outer side.
  • This process is termed as reverse osmosis.
• Potato osmoscope experiment:
  • One peeled potato tuber is taken and made flat at its one end.
  • A deep and broad cavity is made on the other end of it, so that the bed of the cavity may reach up to flat surface, and the outer walls of the cavity becomes somewhat thin.
  • The cavity is filled up with concentrated sugar solution.
  •  The potato is placed in the beaker containing water, such that the level of the outer and inner liquids may remain same.
  • After a short time, it is noted that the level of the liquid filled in the cavity of the potato osmoscope goes higher due to endosmosis.
  • The density of the sugar solution, filled up inside the potato cavity is higher than the water of the beaker.
  • Here, the tissues of the tuber acts as a semipermeable membrane and hence the water moves from outside to inside.
  • Thus, osmosis is demonstrated.

Plasmolysis:

• When protoplast of plant cell is placed in a hypertonic solution, the shrinkage of protoplast takes place due to exo-osmosis. It is termed as plasmolysis.
• The cell soon reaches to its minimum volume after the shrinkage and if the exo-osmosis continues further, the shrinkage stops and protoplasmic membrane begins to recede or contract from the corners first. This stage is termed as incipient plasmolysis.
• If exo-osmosis continues further, there is further contraction of protoplasmic membrane, thus reaching the complete plasmolysis stage.
• In this stage, cell sap, protoplasm and nucleus etc. are wholly contracted in the centre of the cell leaving away the cell wall.
• The sugar or salt solution fills the in-between part of the cell.
• If the plasmolysed cells are placed in water, because of the endosmosis of water, there is recovery to the native condition, and this process is called deplasmolysis.
• Plasmolysis assists to determine nature of membrane, OP of cell, living or dead nature of cells.

3. Imbibition:

• It is a type of diffusion where movement of water takes place along a diffusion gradient.
• Because of this process specific dried and half-dried matters absorb water.
• Substances such as fibres, wood pieces, proteins and sponges are termed as adsorbants.
• An adsorbant is needed for the imbibition to take place.
• The cell wall and protoplasm also absorb water by the process of imbibition.
• The two essential conditions for imbibition to take place are:
• Water potential gradient between surface of adsorbant and the liquid imbibed.
• compatibility between the adsorbant and the imbibed liquid.
• Heat is released during imbibition process i.e. imbibition is an exothermic process.
• Imbibition plays an important role for dry seeds before germination.
• During germination of seeds, water enters the seed first due to imbibition pressure or matrix potential (DPD=IP).

Plant water relations and water potential: Diffusion, Osmosis and Imbibition

The post Plant water relations and water potential: Diffusion, Osmosis and Imbibition appeared first on Online Biology Notes.

• The process of synthesis with the assistance of light is termed as photosynthesis.
• It is the mechanism by which sunlight is used by green plants to produce food from simple molecules like CO2 and H2O.
• Often called carbon assimilation, photosynthesis is expressed by the following equation;
  • 6CO₂ + 6H₂O——— > C₆ H₁₂ O₆ + 6O₂

• The light energy is transformed into chemical energy during the photosynthesis process and is stored into the organic matter, which is normally carbohydrates, and O2 is a by-product of photosynthesis.
• The raw materials for this process are CO₂ and H₂O.
• CO₂ from the air and H₂O from the soil are absorbed.
• The following points make photosynthesis essential to humanity:
• It preserves the atmospheric equilibrium of O₂.
• It provides food either directly as vegetables or indirectly as animal meat or milk which, in turn, is fed to plants.

Where does photosynthesis takes place?

• The photosynthesis occurs in chloroplasts in case of plants and blue-green algae.
• When microscopic observation of a leaf section is done, chlorophyll is not uniformly distributed in the cell.
• However, it is confined to organelles termed as chloroplasts.
• In plants, chloroplasts lie mainly in the cells of mesophyll, a layer that includes several air spaces and a very high concentration of water vapor.
• The exchange of gases takes place between interior of the leaf and the outside through microscopic pores, termed as stomata.
• Each mesophyll cell consists of 20-100 chloroplasts.
• The chloroplast is enclosed by outer and inner membranes.
• The inner membrane encloses a fluid filled region termed as stroma.
• Stroma consists of most of the enzymes needed to produce carbohydrate molecules.
• The third system of membranes is suspended in the stroma.
• This system forms an interconnected set of flat, disc like sacs called thylakoids.
• A fluid-filled internal space, the thylakoid lumen, is enclosed by the thylakoid membrane.
• Thylakoid sacs are arranged in stacks called grana (sing., granum) in some regions of the chloroplast.
•  Each granum is identical to a coin stack, with a thylakoid being each “coin.”
• Some thylakoid membranes extend from one granum to another.
• Thylakoid membranes are involved in ATP synthesis.

Photosynthetic pigments:

• There are major three photosynthetic pigments. They are:
  • Chlorophylls
  • Carotenoid
  • Phycobillins

i. Chlorophylls:

• Chlorophylls are present in thylakoid membranes.
• Thylakoid membranes consist of several kinds of pigments that absorb visible light.
• Various pigments absorb light from various wavelengths.
• Chlorophyll is the dominant photosynthesis pigment and it absorbs light mainly in the visible spectrum’s blue and red areas.
• Green light is not appreciably absorbed by chlorophyll.
• Some of the green light that strikes the plant is either scattered or reflected and hence the plants typically appear to be green in colour.
• There are two principal parts of a chlorophyll molecule, a complex ring and a long side chain.
• A porphyrin ring is ring structure that comprises of smaller rings made of carbon and nitrogen atoms.
• The porphyrin ring absorbs light energy.
• The chlorophyll porphyrin ring is remarkably similar to the red pigment hemoglobin component of the heme in red blood cells.
• However, unlike heme, that holds an atom of iron in the center of the ring, chlorophyll consists of an atom of magnesium in that position.
• The chlorophyll molecule also contains a long, hydrocarbon side chain that makes the molecule extremely nonpolar and ties up the chlorophyll in the membrane
• All chlorophyll molecules in the thylakoid membrane are associated with specific chlorophyll-binding proteins
• Each thylakoid membrane is filled with accurately oriented chlorophyll molecules and chlorophyll-binding proteins that facilitate the transfer of energy from one molecule to another.
• There are many kinds of chlorophyll.
• The chlorophyll ‘a’ is the most essential pigment that starts the light-dependent reactions of photosynthesis.
• Chlorophyll ‘b’ is the accessory pigment which also takes part in photosynthesis.
• It differs from chlorophyll ‘a’ only in a functional group on the porphyrin ring.
• The methyl group (-CH₃) in chlorophyll ‘a’ is replaced in chlorophyll ‘b’ by a terminal carbonyl group (-CHO).
• This difference shifts the wavelengths of light absorbed and reflected by chlorophyll ‘b’.
• It makes it to appear yellow-green, whereas chlorophyll ‘a’ appears bright green.

ii. Carotenoids:

• Chloroplasts have other accessory photosynthetic pigments, such as carotenoids, that are of two types:
  • Carotenes (orange)
  • Xanthophyll (yellow)
• Carotenoids absorb different wavelengths of light than chlorophyll.
• Thus it expands the spectrum of light that provides energy for photosynthesis.
• Chlorophyll may be excited by light directly by energy passed to it from the light source or indirectly by energy passed to it from accessory pigments that have become excited by light.
• When a carotenoid molecule is excited, its energy can be transferred to chlorophyll a.
• In addition, carotenoids are antioxidants that inactivate highly reactive forms of oxygen generated in the chloroplasts hence termed as shield pigment.

iii. Phycobillins:

• It is water soluble pigment.
• It consists of 4 pyrrol rings that lacks Mg and phytol tail.
• It is responsible for maximum absorption in green parts of the spectrum.
• It is distributed in case of red and blue algae.
• Phycoerythrin are distributed in red algae and phycocyanin are distributed in red as well as blue algae.

What are the stages of photosynthesis?

• Photosynthesis consists of two phases.
• Light dependent reaction (grana reaction or Hill reaction or Photochemical reaction or Primary photochemical reaction)
• Dark reactions (light independent reaction or stroma reaction or Blackman’s reaction or carbon-fixation reactions)

Stage I: Light dependent reaction/ Hill reaction:

• Light reaction occurs in grana of chloroplast and it requires the light for it to take place.
• Green plants uses light belonging to visible spectrum (380nm-760nm).
• In light reaction, the light energy is converted into chemical energy in the forms of ATP, NADPH+H⁺, and oxygen.
• According to Robert Hill and Bendall, light reaction comprises of two steps i.e. photolysis and photophosphorylation.
• Light reaction has four successive steps:
  • Absorption
  • Activation
  • Photolysis
  • Photophosphorylation

Photolysis:

• The light reaction initiates when the chlorophyll absorbs light. It causes one of its electrons to move to a higher energy state.
• The excited electron is transferred to an acceptor molecule and is replaced by an electron from water (H₂O).
• It causes the decomposition of water into H⁺ and OH^–, hence termed as photolysis.

Photo-phosphorylation:

• Chlorophylls a and b and accessory pigment molecules are arranged with pigment-binding proteins in the thylakoid membrane into units called antenna complexes.
• The pigments and related proteins are organized as highly ordered groups comprising approximately 250 molecules of chlorophyll associated with particular enzymes and other proteins.
• Each antenna complex absorbs light energy and transfers it to its reaction system.
• This reaction system consists of chlorophyll molecules and proteins, including electron transfer components directly involved in photosynthesis.
• The light energy is converted into chemical energy in the reaction centers by a series of electron transfer reactions.
• Photosynthesis requires two types of photosynthetic units, referred to as photosystem I and photosystem II.
• Their reaction centers can be differentiated as they are related to proteins in a way that induces a slight difference in their absorption spectra.
• Normally, chlorophyll a has a strong absorption peak at about 660 nm.
•  In contrast, the reaction center of photosystem I consists of a pair of chlorophyll a molecules with an absorption peak at 700 nm and is referred to as P700.
• The reaction center of photosystem II is made up of a pair of chlorophyll a molecules with an absorption peak of about 680 nm and is referred to as P680
• Photophosphorylation is further divided into two types:
• i. Non-cyclic photophosphorylation:
  • Photosystem II becomes activated when a pigment molecule in an antenna complex absorbs a photon of light energy.
  • The energy is transferred to the reaction center, where it causes an electron in a molecule of P680 to move to a higher energy level.
  • This energized electron is accepted by a primary electron acceptor (a highly modified chlorophyll molecule known as pheophytin).
  • Then it passes along an electron transport chain until it is donated to P700 in photosystem I.
  • A pigment molecule in an antenna complex associated with photosystem I absorbs a photon of light.
  • The absorbed energy is transferred from one pigment molecule to another until it reaches the reaction center, where it excites an electron in a molecule of P700.
  • This energized electron is transferred to a primary electron acceptor, which is the first of several electron acceptors in a series.
  • The energized electron is passed along an electron transport chain from one electron acceptor to another, until it is passed to ferredoxin, an iron-containing protein.
  • Ferredoxin transfers the electron to NADP⁺ in the presence of the enzyme ferredoxin–NADP⁺ reductase.
  • Although single electrons pass down the electron transport chain, two are needed to reduce NADP⁺. When NADP⁺  accepts 2 electrons, they unite with a proton (H⁺); thus, the reduced form of NADP⁺  is NADPH, which is released into the stroma.
  • P700 becomes positively charged when it gives up an electron to the primary electron acceptor.
  • The missing electron is replaced by one donated by photosystem II.
  • Noncyclic electron transport is a continuous linear process
  • In the availability of light, there is a continuous, one- way flow of electrons from the ultimate electron source, H2O, to the terminal electron acceptor, NADP+.
  • Water goes through enzymatically catalyzed photolysis to replace energized electrons donated to the electron transport chain by molecules of P680 in photosystem II.
  • These electrons travel down the electron transport chain that connects photosystem II with photosystem I.
  • Thus, they provide a continuous supply of replacements for energized electrons that have been given up by P700.
  • As electrons are transferred along the electron transport chain that connects photosystem II with photosystem I, they lose energy.
  • Some of the energy released is used to pump protons across the thylakoid membrane, from the stroma to the thylakoid lumen, producing a proton gradient.
  • The energy of this proton gradient is utilized to produce ATP from ADP by chemiosmosis.
  • ATP and NADPH, the products of the light-dependent reactions, are released into the stroma.
  • Both are required by the carbon fixation reactions.
• ii. Cyclic photophosphorylation:
  • Cyclic electron transport is the simplest light-dependent reaction.
  • Only Photosystem I is involved in this reaction.
  • The term cyclic is given for the pathway as the energized electrons that initiate from the P700 at the reaction finally return to P700.
  • In the presence of light, electrons flow continuously through an electron transport chain within the thylakoid membrane.
  • As the electrons pass from one acceptor to the other, the electrons lose energy.
  • Some of the energy is used to pump protons across the thylakoid membrane.
  • ATP synthase, an enzyme present in the thylakoid membrane uses the energy of the proton gradient to manufacture ATP.
  • In the cyclic photophosphorylation, NADPH is not produced, H₂O is not split, and oxygen is not generated.
  • Hence, it cannot serve as basis for photosynthesis as NADPH is required to reduce CO₂ to carbohydrate.
  • The importance of cyclic electron transport to photosynthesis in plants is unclear.
  • Cyclic electron transport may take place in plant cells when there is too little NADP+ to accept electrons from ferredoxin.
  • There is a proof that cyclic electron flow may aid to maintain the optimal ratio of ATP to NADPH required for carbon fixation as well as provide extra ATP to power other ATP-requiring processes in chloroplasts.
  • The process is termed as photophosphorylation as the synthesis of ATP (i.e. the phosphorylation of ADP) is coupled to the transport of electrons that have been energized by photons of light.

Stage II: Dark reaction/Carbon fixation reactions:

• In carbon fixation, the energy of ATP and NADPH is used in the formation of organic molecules from CO₂ in carbon fixation.
• The carbon fixation reactions may be summarized as follows:
• 12NADPH + 18ATP + 6CO₂ —> C₆H₁₂O₆ + 12NADP⁺ +18ADP +18P_i+ 6H₂O
• Carbon fixation occurs in the stroma of chloroplast and is again classified into two types:
  • Calvin cycle/C₃ cycle
  • Hatch and Slack pathway or C₄ cycle

i. Calvin cycle/C₃ cycle:

• It comprises a series of 13 reactions.
• These reactions of the Calvin cycle are divided into three phases:
  • Uptake of CO₂
  • Carbon reduction
  • RuBP regeneration
• All 13 enzymes that catalyze steps in the Calvin cycle are located in the stroma of the chloroplast.
• These enzymes catalyze reversible reactions, degrading carbohydrate molecules in cellular respiration and synthesizing carbohydrate molecules in photosynthesis.
• Uptake of CO₂:
  • The first phase of the Calvin cycle consists of a single reaction.
  •  In this reaction, a molecule of CO2 reacts with a phosphorylated 5-carbon
    compound, ribulose bisphosphate (RuBP).
  • This reaction is catalyzed by the enzyme ribulose bisphosphate carboxylase/oxygenase, also termed as rubisco.
  • The chloroplast contains abundant rubisco enzyme than any other protein.
  • The product of this reaction is an unstable 6-carbon intermediate, which instantly breaks down into 2 molecules of phosphoglycerate (PGA) with 3 carbons each.
  • The carbon that was initially the part of a CO2 molecule is now part of a carbon skeleton; i.e. the carbon has been “fixed.”
  • The Calvin cycle is also known as the C3 pathway asthe product of the initial carbon fixation reaction is a 3-carbon compound.
  • Plants that initially fix carbon in this way are called C3 plants.
•  Carbon reduction:
  • The second phase of the Calvin cycle involves two steps in which the energy and reducing power from ATP and NADPH (both produced in the light-dependent reactions) are used to convert the PGA molecules to glyceraldehyde-3-phosphate (G3P).
  • For every 6 carbons that enter the cycle as CO2, 6 carbons can leave the system as 2 molecules of G3P, to be used in carbohydrate synthesis.
  • Each of these 3-carbon molecules of G3P is essentially half a hexose (6-carbon sugar) molecule.
  • The reaction of 2 molecules of G3P is exergonic and leads to the formation of glucose or fructose.
  • In some plants glucose and fructose are then joined to produce sucrose
  • The plant cell also uses glucose to produce starch or cellulose.
•  RuBP regeneration:
  • Even if, two G3P molecules are eliminated from the cycle, 10 G3P molecules remains, this represents a total of 30 carbon atoms.
  • Through a series of 10 reactions that make up the third phase of the Calvin cycle, these 30 carbons and their associated atoms become rearranged into 6 molecules of ribulose phosphate.
  • Each of them becomes phosphorylated by ATP to produce RuBP, the 5-carbon compound with which the cycle started.
  • These RuBP molecules begin the process of CO2 fixation and eventual G3P production once again.
  • In summary, the inputs required for the carbon fixation reactions are 6 molecules of CO2, phosphates transferred from ATP, and electrons (as hydrogen) provided by NADPH (but ultimately derived from the photolysis of water).
  • In the end, the 6 carbons from the CO2 are accounted for by the harvest of a hexose molecule.
  • The remaining G3P molecules are used to synthesize the RuBP molecules with which more CO2 molecules may combine.

ii. Hatch and slack pathway/ C₄ cycle:

• Several plant species found in hot, dry environments have adaptations that facilitate carbon fixation.
• C4 plants first fix CO2 into a 4-carbon compound, oxaloacetate.
• CAM plants initially fix carbon at night through the formation of oxaloacetate.
• These special pathways found in C4 and CAM plants precede the Calvin cycle (C3 pathway); they do not replace it.
• C₄ pathway:
• The C4 pathway efficiently fixes CO2 at low concentrations.
• In this pathway, CO2 is fixed through the formation of oxaloacetate, occurs not only before the C3 pathway but also in different cells.
• Leaf anatomy is usually typical in C4 plants.
• The photosynthetic mesophyll cells are closely associated with prominent, chloroplast-containing bundle sheath cells, which tightly surrounds the veins of the leaf.
• The C4 pathway occurs in the mesophyll cells, whereas the Calvin cycle takes place within the bundle sheath cells.
• The major component of the C4 pathway is PEP carboxylase.
• It is a peculiar enzyme that has an extremely high affinity for CO2 i.e. binds it effectively even at unusually low concentrations.
• Carboxylation:
  • PEP carboxylase catalyzes the reaction by which CO2 reacts with the 3-carbon compound phosphoenolpyruvate (PEP) and forms oxaloacetate.
• Breakdown:
  • In a step that needs NADPH, oxaloacetate is converted to some other 4-carbon compound, malate and aspartate in presence of enzyme transaminase and malate dehydrogenase.
• Splitting:
  • The malate and aspartate then passes to chloroplasts within bundle sheath cells.
  • Here, a different enzyme catalyzes the decarboxylation of malate and yields pyruvate (which has 3 carbons) and CO2.
  • NADPH is formed, replacing the one used earlier.
  • ^{Malate + NADP+}  —->  ^{pyruvate + CO2 + NADPH}
  • The CO2 released in the bundle sheath cell combines with ribulose bisphosphate in a reaction catalyzed by rubisco and undergoes the Calvin cycle in the usual manner.
• Phosphorylation:
  • The pyruvate formed in the decarboxylation reaction returns to the mesophyll cell, where it reacts with ATP to regenerate phosphoenolpyruvate.
  • As the C4 pathway captures CO2 and provides it to the bundle sheath cells so efficiently, CO2 concentration within the bundle sheath cells is about 10 to 60 times as great as its concentration in the mesophyll cells of plants having only the C3 pathway.
  • Photorespiration is negligible in C4 plants such as crabgrass because the concentration of CO2 in bundle sheath cells (where rubisco is present) is always high.
  • The combined C3-C4 pathway involves the expenditure of 30 ATPs per hexose rather than the 18 ATPs used by the C3 pathway alone.
  • The extra energy expense required to regenerate PEP from pyruvate is worthwhile at high light intensities because it ensures a high concentration of CO2 in the bundle sheath cells and allows them to carry on photosynthesis at a rapid rate.
  • At lower light intensities and temperatures, C3 plants are favored.
  • For example, winter rye, a C3 plant, grows lavishly in cool weather, while crabgrass cannot because it requires more energy to fix CO2.

Fixation of  CO₂ by CAM plants at night:

• Plants living in dry, or xeric, conditions have a number of structural adaptations that enable them to survive.
• Many xeric plants have physiological adaptations as well, including a special carbon fixation pathway termed as the crassulacean acid metabolism (CAM) pathway.
• Unlike most plants, CAM plants open their stomata at night, permitting CO2 while minimizing water loss.
• They use the enzyme PEP carboxylase to fix CO2, forming oxaloacetate, which is converted to malate and stored in cell vacuoles.
• During the day, when stomata are closed and gas exchange cannot take place between the plant and the atmosphere, CO2 is removed from malate by a decarboxylation reaction.
• Now the CO2 is available within the leaf tissue to be fixed into sugar by the Calvin cycle (C3 pathway).
• The CAM pathway is very similar to the C4 pathway but with important differences.
• C4 plants initially fix CO2 into 4-carbon organic acids in mesophyll cells.
• The acids are later decarboxylated to produce CO2, which is fixed by the C3 path- way in the bundle sheath cells.
• In other words, the C4 and C3 pathways occur in several locations within the leaf of a C4 plant.
• In CAM plants the initial fixation of CO2 occurs at night.
• Decarboxylation of malate and subsequent production of sugar from CO2 by the normal C3 photosynthetic pathway occur during the day.
• In other words, the CAM and C3 pathways occur at various times within the same cell of a CAM plant.
• Although it does not promote rapid growth the way that the C4 pathway does, the CAM pathway is a very successful adaptation to xeric conditions.
• CAM plants can exchange gases for photosynthesis and reduce water loss significantly.
• Plants with CAM photosynthesis survive in deserts where neither C3 nor C4 plants can.

Photosynthesis: Definition, photosynthetic pigments, stage of light and dark reaction

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• Meristem is responsible for the development of primary plant body.
• Primary growth increases length of the plant as well as lateral appendages.
• However, secondary Grier increases thickness or girth of the plant by the formation of secondary tissues.
• There secondary tissues are formed by the two types of lateral meristem i.e. vascular cambium and cork cambium (phellogen).
• Secondary growth occurs in stem and root of dicots and gymnosperms.
• However, it is absent in stem and root of monocot and completely absent in leaf.
• A process of formation of secondary tissues due to activity of vascular cambium and cork cambium for increasing thickness or girth or diameter of plant is termed as secondary growth.
• On the basis of the activities of vascular cambium and cork cambium, the process of secondary growth can be discussed under the following headings:
  • Activity of the vascular cambium
  • Activity of the cork-cambium

Secondary growth in stellar region due to activity of the vascular cambium

i. Formation of cambium ring:

• In vascular bundles of a dicot stem, the cambium is present in between the xylem and phloem. It is known as intrafascicular cambium.
• During secondary growth, some cells of medullary rays become active and show meristematic activity which form a strip of cambium in between vascular bundles called inter-fascicular cambium.
• Both the intra-fascicular and inter-fascicular cambium unite together to form a complete ring called the cambium ring.
• The activity of the cambium ring gives rise to secondary growth.

ii. Formation of the secondary tissues:

• The cambium ring acts as a meristem which divides.
• The cambium layer consists of a single layer of cells.
• These cells divide in a direction parallel with epidermis.
• A cambial cell divides into two daughter cells, one of which remains meristematic and other differentiates into secondary vascular tissue.
• The cell formed towards inner side develops into secondary xylem.
• Likewise, the cell formed towards outer side develops into secondary phloem.
• Normally, more secondary xylem cells are formed towards the center due to which cambium ring moves towards the periphery.
• Due to the formation of secondary xylem and secondary phloem, the primary xylem and primary phloem which were initially closed, moves towards inner and outer side respectively.
• As a result, they become separated apart.
• The layers of secondary tissues gradually added to the inner and outer side of the cambium continuously throughout the life of the plant.

iii. Formation of secondary medullary rays:

• Certain cells of the cambium instead of forming secondary xylem and phloem for some narrow bands of living parenchyma cells.
• These form two or three layers of thick radical rows of cells passing through the secondary xylem and secondary phloem and are called secondary medullary rays.
• These provide the radial conduction of food from the phloem, and water and mineral salts from the xylem.

iv. Formation of annual rings:

• The activity of cambium is affected by variations in temperature.
• In moderate climate, the cambium becomes more active in the spring and forms greater number of vessels with wider cavities, whereas in winter it becomes less active and forms narrower and smaller vessels.
• The wood formed in the spring is known as spring wood or early wood and that formed in the dry summer or cold winter is autumn wood or late wood.
• These two kinds of wood appear together as a concentric ring known as the annual ring or growth ring, as seen in transection of the stem and successive annual rings are formed year after year by the activity of the cambium.
• The growth of the successive years appears in the form of concentric or annual rings, each annual ring representing the one year’s growth.
• The age of the plant thus, can be approximately determined by counting the number of annual rings.

v. Formation of heart wood and sap wood:

• In the old trees, where sufficient amount of secondary growth has taken place, the secondary wood of inner side lose the power of conduction.
• Their cells get filled with tannins, resins, gums, essential oils which makes the plant part hard and darker called the heart wood or duramen.
• The heart wood ceases the function of conducting tissue and simply provides mechanical support to the stem.
• However, the outer region of secondary wood, which consists of younger living xylem cells, remains yellow in colour called the sap wood or laburnum.
• It functions as the conducting tissue and also as the food storage tissue.

Secondary growth in extra stellar region due to activity of cork-cambium:

• The marked increase in diameter or thickness of stem brought about by the secondary thickening exerts a great pressure on the outer tissues.
• This results in the rupture of the cortex and epidermis, the outer cortical cells become meristematic and begins to divide. This is known as cork cambium or phellogen.
• The cork cambium divides to form secondary tissue on both the sides i.e. internal and external but its activity is more on the outer side than on the inner side.
• The cells formed on the outer side constitutes the phellem or cork and those on the inner side form secondary cortex or phelloderm.
• The phellogen, phellem and phelloderm together are called periderm.

Secondary growth in dicot stem

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T.S. of monocot root shows the following anatomical features:

Epidermis/Epiblema/Rhizodermis:

• It is the outermost layer composed of compact parenchymatous cells having no intercellular spaces and stomata.
• The tubular unicellular root hairs are also present on this layer
• Both epiblema and root hairs are without cuticle.
• In older parts, epiblema either becomes impervious or is shed.
• Epiblema and root hairs absorb water and mineral salts.

Cortex:

• It lies just below the epidermis.
• Cortex consists of thin walled multilayered parenchyma cells having sufficiently developed intercellular spaces among them.
• Usually in an old root of Zea mays, a few layers of cortex undergo suberization and give rise to a single or multi-layered zone- the exodermis.
• This is a protective layer which protects internal tissues from outer injurious agencies.
• The starch grains are abundantly present in the cortical cells.
• Cortex functions as:
  • a) conduction of water and mineral salts from root hairs to inner tissues
  • b) storage of food
  • c) protection when exodermis is formed in older parts.

Endodermis:

• The innermost layer of the cortex is termed as endodermis.
• It is composed of barrel-shaped compact cells that lacks intercellular spaces among them.
• Young endodermal cells have an internal strip of suberin and lignin which is called casparian strip.
• The strip is located close to the inner tangential wall.
• There are some unthickened cells opposite to the protoxylem vessels known as passage cells which serve for conducting of fluids.
• The function of endodermis is to regulate the flow of both inward as well as outward.

Pericycle:

• It lies just below the endodermis and is composed of single layered sclerenchymatous cells intermixed with parenchyma.

Vascular tissue:

• The vascular tissue contains alternating strands of xylem and phloem.
• The phloem is visualized in the form of strands near the periphery of the vascular cylinder, beneath the pericycle.
• The xylem forms discrete strands, alternating with phloem strands.
• The center is occupied by large pith which maybe parenchymatous or sclerenchymatous.
• The number of vascular bundles is more than six, hence called as polyarch.
• Xylem is exarch i.e. the protoxylem is located towards the periphery and the metaxylem towards the center.
• Vessels of protoxylem are narrow and the walls possess annular and spiral thickenings in contrast, metaxylem are broad and the walls have reticulate and pitted thickenings.
• Phloem strands consist of sieve tubes, companion cells and phloem parenchyma.
• The phloem strands are also exarch having protophloem towards the periphery and metaphloem towards the center.

Conjunctive tissues:

• In between the xylem and phloem bundles, there is the presence of many layered parenchymatous or sclerenchymatous tissue.
• These help in storage of food and help in mechanical support.

Pith:

• It is the central portion usually composed of thin-walled parenchymatous cells which appear polygonal or rounded in T.S.
• Intercellular spaces may or may not be present amongst pith cells.
• In some cases pith becomes thick walled and lignified.
• Pith cells serve to store food.

Internal structure of Monocot root

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T.S. of a monocot stem shows the following anatomical features:

Epidermis:

• It is the single outermost layer composed of small, thin-walled, somewhat barrel-shaped parenchymatous cells which are tightly packed without intercellular species.
• It is externally covered with thick cuticle.
• A few stomata are present on epidermis.
• Usually trichomes or hairs are lacking.

Cortex:

• It lies below the epidermis.
• Cortex is composed of following regions:
• Hypodermis:
• It lies just below the epidermis.
• It comprises of 2-3 layers of thick-walled lignified sclerenchymatous cells, without intercellular spaces.
• It helps in mechanical support.

Ground tissue:

• It contains a continuous mass of thin-walled, round parenchymatous tissue which lies below the hypodermis.
• The intercellular spaces are present.
• Cells are rounded or polygonal in shape.
• There is no differentiation of general cortex, endodermis, pericycle, pith, and rays.
• Vascular bundles are irregularly lodged in this region.

Vascular bundles:

• Vascular bundles are irregularly scattered in the ground tissues, called atactostele.
• These are conjoint, collateral, and closed type.
• Vascular bundles occurring in the peripheral region are smaller in size and compactly arranged.
• In contrast, those occurring towards the central region are larger in size and widely placed.
• All the vascular bundles have similar structure.
• Each vascular bundle consists of xylem towards the center and phloem towards the periphery without cambium.
• It is oval in shape and surrounded by a sheath of sclerenchymatous tissue.

Xylem:

• It is V or Y shaped, bearing two large metaxylem vessels with wider cavities and pitted thickening at the lateral arms.
• Few tracheids are present in between the metaxylem vessels.
• Protoxylem vessels are only one or two, smaller, narrow cavities having annular or formed lysigenously by disintegrating by disintegration or breaking of some cells or parenchyma tissue and rarely protoxylem vessels.
• Thin-walled xylem parenchyma is present around the protoxylem vessels.

Phloem:

• It lies outside the xylem and is partly present near the metaxylem vessels.
• It is composed of sieve elements and companion cells.
• In a mature bundle, the protophloem gets crushed just below the sheath. So, the inner portion is meta-phloem.
• Sieve tubes appear polygonal in T.S. having internally situated companion cells.
• It conducts the organic food.

Internal structure of Monocot stem

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T.S. of dicot stem shows following internal features:

Epidermis:

• It is the outermost layer and has a single layer of parenchymatous cells.
• It possesses stomata and large number of multicellular hairs (trichomes).
• The outer walls are greatly thickened and cutinized.
• The cells are compactly arranged and do not possess intercellular space.
• The epidermis has following functions:
• Minimize the rate of transpiration owing thick cuticle
• Protects the underlying tissues from mechanical injury
• Prevents the entry of harmful organisms
• Helps in the exchange of gases through stomata.

Hypodermis:

• This layer lies below the epidermis and is composed of 4 or 5 layers of collenchymatous cells.
• These cells are specially thickened at the corners against the intercellular spaces due to deposition of cellulose and pectin.
• The cells are living in nature and may contain few chloroplasts.
• It provides mechanical strength and elasticity to the peripheral portion of the stem particularly the young and growing organs.
• They perform photosynthesis and also acts as storage of food.

Cortex:

• It lies below the hypodermis.
• It consists of a few layers of thin-walled, large, rounded, or oval, living parenchymatous cells, having intercellular spaces.
• Cells of cortex may contain some chloroplasts which may function to manufacture of food materials.
• They serve for storage of food.

Endodermis:

• It is the single innermost layer of the cortex which separates the cortex from vascular bundles.
• Cells are somewhat barrel shaped and compactly arranged, having no intercellular spaces and are parenchymatous.
• Usually, the cells contain starch grains and thus the endodermis maybe termed as starch sheath.
• They serve as food reserve.
• The radial and the transverse walls are thickened due to the deposition of lignin forming casparian strips.

Pericycle:

• It lies in between the endodermis and vascular bundles.
• It is generally composed of sclerenchymatous and parenchymatous cells.
• The sclerenchyma is in the form of semilunar patches above the vascular bundles which give mechanical support to the plant parts.
• Similarly, parenchymatous pericycle is present outside the medullary rays which serves to store food.

Vascular bundles:

• These are arranged in a ring around the central pith and inner to the pericycle.
• These are conjoint, collateral, open and wedge-shaped.
• The size of the bundles varies in different species.
• Each bundle has a patch of xylem towards the center, a patch of phloem towards the periphery and a strip of cambium in between them.
• Xylem:
  • It lies towards the pith of vascular bundles.
  • It consists of tracheids, vessels, xylem parenchyma, xylem fibers.
  • Tracheids and vessels consists of smaller protoxylem and larger metaxylem.
  • Protoxylem is first formed that lies towards the center but metaxylem is later formed that lies towards the periphery.
  • This type of xylem is called endarch xylem. It helps in conduction of sap.
• Phloem:
  • It lies just below the sclerenchymatous patch of pericycle and is composed of following elements such as sieve tubes, companion cells, and phloem parenchyma.
  • It conducts the foods.
  • Cambium:
  • It lies in between xylem and phloem.
  • It consists of a narrow strip of meristematic cells having large nuclei and dense cytoplasm, called fascicular cambium.
  • It is responsible for secondary growth in thickness of the plant body.

Pith:

• It occupies the central portion of the stem.
• It is composed of thin walled parenchymatous cells which are rounded or polygonal, with or without intercellular spaces.
• Food is stored in this region.
• Medullary rays:
• These are the thin-walled, radially elongated parenchymatous cells present in between vascular bundles.
• These store food materials and help in internal translocation of water.

Internal structure of dicot stem

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