Control Of Growth Responses In Plants PDF

1. Differentiation This ability is called plasticity. Example: The process of maturation of meristematic Heterophylly in cotton and coriander. cells to specific types of cells performing In such plants, the leaves of the juvenile specific f...

1. Differentiation This ability is called plasticity. Example: The process of maturation of meristematic Heterophylly in cotton and coriander. cells to specific types of cells performing In such plants, the leaves of the juvenile specific functions is called differentiation. plant are different in shape from those in mature plants. On the other hand, 2. Dedifferentiation the difference in shapes of leaves The living differentiated cells which had produced in air and those produced in lost capacity to divide, regain the capacity water in buttercup also represent the to divide under certain conditions. Hence, heterophyllous development due to dedifferentiation is the regaining of the the environment. This phenomenon of ability of cell division by the differentiated heterophylly is an example of plasticity. cells. Example: Interfascicular cambium and Vascular cambium. 15.2 Plant Growth Regulators 3. Redifferentiation Plant Growth Regulators Differentiated cells, after multiplication (chemical messenger) again lose the ability to divide and mature are defined as organic to perform specific functions. This is called substances which are redifferentiation (Figure 15.9). Example: synthesized in minute Secondary xylem and Secondary phloem. quantities in one part of the plant body and transported to 4. Plasticity another part where they influence specific Plants follow different pathways in physiological processes. Five major groups response to environment or phases of of hormones viz., auxins, gibberellins, life to form different kinds of structures. cytokinins, ethylene and abscisic acid are presently known to coordinate and regulate growth and development in plants. The term phytohormones is implied to those chemical substances which are synthesized by plants and thus, naturally occurring. On the other hand, there are several manufactured chemicals which often resemble the hormones in physiological action and even in molecular structure. Recently, another two groups, the brassinosteroids and polyamines were also known to behave like hormones. 1. Plant growth regulators – classification Plant Growth Regulators are classified Figure 15.9: Sequences of developmental process in a plant cell as natural and synthetic based on their 175  Plant Growth Regulators (PGRs) Natural (Phytohormones) Synthetic Plant Growth Promoters Growth inhibitors Auxin Ethylene NAA Gibberellin Abscisic acid 2,4 -D Cytokinin 2,4,5 - T Figure 15.10: Classification of Plant Growth Regulators source and a detailed flow diagram is ii. Antagonistic effects: The effect of two given in Figure 15.10. substances in such a way that they have opposite effects on the same process. 2. Characteristics of phytohormones One accelerates and other inhibits. i. Usually produced in tips of roots, stems Example: ABA and gibberellins during and leaves. seed or bud dormancy. ABA induces ii. Transfer of hormones from one place to dormancy and gibberellins break it. another takes part through conductive systems. 15.2.1 Auxins iii. They are required in trace quantities. 1. Discovery iv. All hormones are organic in nature. During 1880, Charles Darwin noted the v. There are no specialized cells or organs unilateral growth and curvature of Canary for their secretion. grass (Phalaris canariensis) coleoptile to light. vi. They are capable of influencing The term auxin (Greek: Auxin – to Grow) physiological activities leading to was first used by F. W. Went in 1926 using promotion, inhibition and modification Oats (Avena) coleoptile and isolated the of growth. auxin. F. W. Went in 1928 collected auxin in agar jelly. Kogl and Haugen Smith (1931) 3. Synergistic and Antagonistic effects isolated Auxin from human urine, and called i. Synergistic effects: The effect of one or it as Auxin A. Later on in 1934, similar active more substance in such a way that both substances was isolated from corn grain oil promote each others activity. Example: and was named as Auxin B. Kogl et al., (1934) Activity of auxin and gibberellins or found heteroauxin in the plant and chemically cytokinins. called it as Indole Acetic Acid (IAA) 176  Types of Auxin Natural Synthetic Auxin occuring in plants are called These are synthesized artificially and have “Natural auxin” properties like Auxin. 1. Indole Acetic Acid (IAA) 1. 2,4-Dichloro Phenoxy Acetic Acid (2,4-D) 2. Indole Propionic Acid (IPA) 2. 2,4,5-Trichloro Phenoxy Acetic Acid (2,4,5-T) 3. Indole Butyric Acid (IBA) 3. Napthalene Acetic Acid (NAA) 4. Phenyl Acetic Acid (PAA) Figure 15.11: Classification of Auxins 2. Occurrence 7. Chemical structure Auxin is generally produced by the growing Auxin has similar chemical structure of tips of the stem and root, from where they IAA. migrate to the region of the action. 8. Transport in Plants 3. Types of Auxin Auxin is polar in transport. It includes Auxins are divided into two categories basipetal and acropetal transport. Natural auxins and Synthetic auxins Basipetal means transport through (Figure 15.11). phloem from shoot to root and acropetal means transport through xylem from root Anti-auxins to shoot. Anti-auxin compounds when applied to the plant inhibit the effect of auxin. 9. Bioassay (Avena Curvature Test / Example: 2, 4, 5-Tri Iodine Benzoic Went Experiment) Acid (TIBA) and Napthylpthalamine. Bioassay means testing of substances for their activity in causing a growth response 4. Free auxin in a living plant or its part. They move out of tissues as they are easily The procedure involves the following diffusible. Example: IAA. steps: 5. Bound Auxin When the Avena seedlings have attained They are not diffusible. Example: IAA- a height of 15 to 30 mm, about 1mm of Aspartic acid the coleoptile tip is removed. This apical part is the source of natural auxin. The 6. Precursor tip is now placed on agar blocks for few The amino acid Tryptophan is the hours. During this period, the auxin precursor of IAA and zinc is required for diffuses out of these tips into the agar. its synthesis. The auxin containing agar block is now 177  Auxin in the Auxin containing agar block Diffusion of Auxin Avena coleoptile in one side of stump from agar block Decapited stump Coleoptile placed on Agar Block Auxin diffuses in to agar block Figure 15.12: Avena Curvature Test placed on one side of the decapitated and for the formation of callus. stump of Avena coleoptile. The auxin Auxin stimulates respiration. from the agar blocks diffuses down Auxin induces vascular differentiation. through coleoptile along the side to which the auxin agar block is placed. An Agent Orange agar block without auxin is placed on another decapitated coleoptile. Within Mixture of two phenoxy herbicides an hour, the coleoptiles with auxin agar 2,4-D and 2,4,5-T is given the name block bends on the opposite side where ‘Agent orange’ which was used by the agar block is placed. This curvature USA in Vietnam war for defoliation can be measured (Figure 15.12). of forest (chemical warfare). 10. Physiological Effects They promote cell elongation in stem and coleoptile. At higher concentrations auxins inhibit the elongation of roots but induce more lateral roots. Promotes growth of root only at extremely low concentrations. Suppression of growth in lateral bud by apical bud due to auxin produced by apical bud is termed as In botanical gardens and tea gardens, apical dominance. gardeners trim the plants regularly Auxin prevents abscission. so that they remain bushy. Does this It is responsible for initiation and practice have any scientific explanation? promotion of cell division in cambium, Yes, trimming of plants removes which is responsible for the secondary apical buds and hence apical growth and tumor. This property of dominance. The lateral buds sprout induction of cell division has been and make the plants bushy. exploited for tissue culture techniques 178 11. Agricultural role and steroids) formed by 5-C precursor, It is used to eradicate weeds. Example: an Isoprenoid unit called Iso Pentenyl 2,4-D and 2,4,5-T. Pyrophosphate (IPP) through a number Synthetic auxins are used in the formation of intermediates. The primary precursor of seedless fruits (Parthenocarpic fruit). is acetate. It is used to break the dormancy in seeds. 4. Chemical structure Induce flowering in Pineapple by NAA All gibberellins have gibbane ring & 2,4-D. structure. Increase the number of female flowers and fruits in cucurbits. 5. Transport in plants The transport of gibberellins in plants is 15.2.2 Gibberellins non-polar. Gibberellins are translocated through phloem and also occur in xylem 1. Discovery due to lateral movement between vascular The effect of gibberellins had been known bundles. in Japan since early 1800 where certain rice plants were found to suffer from 6. Bioassay (Dwarf Pea assay) ‘Bakanae’ or foolish seedling disease. Seeds of dwarf pea are allowed to germinate This disease was found by Kurosawa till the formation of the coleoptile. GA (1926) to be caused by a fungus Gibberella solution is applied to some seedlings. Others fujikuroi. The active substance was are kept under control. Epicotyle length separated from fungus and named as is measured and as such, GA stimulating gibberellin by Yabuta (1935). These are epicotyle growth can be seen. more than 100 gibberellins reported from both fungi and higher plants. They are 7. Physiological Effects noted as GA1, GA2, GA3 and so on. GA3 It produces extraordinary elongation is the first discovered gibberellin. In 1938, of stem caused by cell division and cell Yabuta and Sumiki isolated gibberellin in elongation. crystalline form. In1955, Brain et al., gave Rosette plants (genetic dwarfism) the name gibberellic acid. In 1961, Cross plants exhibit excessive internodal et al., established its structure. growth when they are treated with gibberellins. This sudden elongation 2. Occurrence of stem followed by flowering is called The major site of gibberellin production bolting (Figure 15.13). in plants is parts like embryo, roots and Gibberellin breaks dormancy in potato young leaves near the tip. Immature seeds tubers. are rich in gibberellins. Many biennials usually flower during second year of their growth. For 3. Precursors flowering to take place, these plants The gibberellins are chemically related to should be exposed to cold season. Such terpenoids (natural rubber, carotenoids plants could be made to flower without 179  (liquid endosperm of coconut) which contains cell division inducing substances. In 1954, Skoog and Miller discovered that autoclaved DNA from herring Rosette leaves sperm stimulated cell division in tobacco pith cells. They called this cell division inducing principle as kinetin (chemical structure: 6-Furfuryl Amino Acid). (a) Untreated plant (b) Treated plant This does not occur in plants. In 1963, showing bolting. Lethan introduced the term cytokinin. Figure 15.13: Bolting In 1964, Lethan and Miller isolated and identified a new cytokinin called Zeatin exposure to cold season in the first from unripe grains of maize. The most year itself, when they are treated with widely occurring cytokinin in plants is gibberellins. Iso Pentenyl adenine (IPA). 8. Agricultural role 2. Occurrence Formation of seedless fruits without Cytokinin is formed in root apex, shoot fertilization is induced by gibberellins apex, buds and young fruits. Example: Seedless tomato, apple and cucumber. 3. Precursor It promotes the formation of male Cytokinins are derivatives of the purine flowers in cuccurbitaceae. It helps in adenine. crop improvement. 4. Bioassay (Neem Cotyledon Assay) Uniform bolting and increased uniform seed production. Neem cotyledons are measured and placed in cytokinin solution as well as in ordinary Improves number and size of fruits in water. Enlargement of cotyledons is an grapes. It increase yield. indication of cytokinin activity. Promotes elongation of inter-node in sugarcane without decreasing sugar 5. Transport in plants content. The distribution of cytokinin in plants Promotion of flowering in long day is not as wide as those of auxin and plants even under short day conditions. gibberellins but found mostly in roots. It stimulates the seed germination. Cytokinins appear to be translocated through xylem. 15.2.3 Cytokinins (Cytos – cell, 6. Physiological effect Kinesis – division) Cytokinin promotes cell division in the 1. Discovery presence of auxin (IAA). The presence of cell division inducing Induces cell enlargement associated substances in plants was first demonstrated with IAA and gibberellins by Haberlandt in 1913 in Coconut milk 180  Cytokinin can break the dormancy 4. Precursor of certain light-sensitive seeds like It is a derivative of amino acid methionine, tobacco and induces seed germination. linolenic acid and fumaric acid. Cytokinin promotes the growth of lateral bud in the presence of apical bud. 5. Bioassay (Gas Chromatography) Ethylene can be measured by gas Application of cytokinin delays the chromatography. This technique helps in process of aging by nutrient mobilization. the detection of exact amount of ethylene It is known as Richmond Lang effect. from different plant tissues like lemon Cytokinin (i) increases rate protein and orange. synthesis (ii) induces the formation of inter-fascicular cambium 6. Physiological Effects (iii) overcomes apical dominance Ethylene stimulates respiration and (iv) induces formation of new leaves, ripening in fruits. chloroplast and lateral shoots. It stimulates radial growth in stem and Plants accumulate solutes very actively root and inhibits linear growth. with the help of cytokinins. It breaks the dormancy of buds, seeds and storage organs. 15.2.4 Ethylene It stimulates formation of abscission (Gaseous Phytohormone) zone in leaves, flowers and fruits. This makes the leaves to shed prematurely. Almost all plant tissues produce ethylene Inhibition of stem elongation gas in minute quantities. (shortening the internode). 1. Discovery In low concentration, ethylene helps in In 1924, Denny found that ethylene stimulates root initiation. the ripening of lemons. In 1934, R. Gane Growth of lateral roots and root hairs. found that ripe bananas contain abundant This increases the absorption surface of ethylene. In 1935, Cocken et al., identified the plant roots. ethylene as a natural plant hormone. The growth of fruits is stimulated by ethylene in some plants. It is more 2. Occurrence marked in climacteric fruits. Maximum synthesis occurs during Ethylene causes epinasty. climacteric ripening of fruits (see Box info) and tissues undergoing senescence. Agricultural role It is formed in almost all plant parts like Ethylene normally reduces flowering in roots, leaves, flowers, fruits and seeds. plants except in Pine apple and Mango. It increases the number of female 3. Transport in plants flowers and decreases the number of Ethylene can easily diffuse inside the plant male flowers. through intercellular spaces. Ethylene spray in cucumber crop produces female flowers and increases the yield. 181  Ethylene Synergistic effects Auxin Auxin, GA3 and Cytokinin induces X X x Plant growth Fruit InduceEthylene Apical Prevents Abscission ripening dominance abscission Weedicide Plant Growth Cytokinins Regulators 182 12 9 3 Radial growth 6 x ABA Root /Shoot Delaying Promote initiation from ageing lateral bud ABA GA3 Callus process growth ABA Induces Breaks Gibberellins seed seed ABA dormancy dormancy Yellowing Induce Closure of GROWTH PROMOTERS GROWTH INHIBITORS of leaf Abscission stomata Antagonistic effects Bolting  3. Precursors Climacteric fruits: In most of the The hormone is formed from mevalonic plants, there is sharp rise in respiration acid pathway or xanthophylls. rate near the end of the development of fruit, called climacteric rise. Such 4. Transport in plants fruits are called climacteric fruits. The Abscisic acid is transported to all parts ripening on demand can be induced of the plant through diffusion as well as in these fruits by exposing them to through phloem and xylem. normal air containing about 1 ppm 5. Chemical structure of ethylene. A liquid called ethephon is being used in fruit ripening as it It has carotenoid structure. continuously releases ethylene. 6. Bioassay (Rice Coleoptile) Example: Tomato, Apples, Banana, The inhibition of IAA induces straight Mango. growth of rice seedling coleoptiles. Non climacteric fruits: All fruits 7. Physiological effects cannot be ripened by exposure to ethylene. Such fruits are called non- It helps in reducing transpiration rate climacteric fruits and are insensitive by closing stomata. It inhibits K1 uptake to ethylene. by guard cells and promotes the leakage of malic acid. It results in closure of Example: Grapes, Watermelon, stomata. Orange. It spoils chlorophylls, proteins and nucleic acids of leaves making them 15.2.5 Abscisic Acid (ABA) yellow. (Stress Phyto Hormone) Inhibition of cell division and cell 1. Discovery elongation. In 1963, the hormone was first isolated ABA is a powerful growth inhibitor. It by Addicott et al., from young cotton causes 50% inhibition of growth in Oat bolls and named as Abscission II. Eagles coleoptile. and Wareing during 1963–64 isolated a It induces bud and seed dormancy. dormancy inducing substance from leaves It promotes the abscission of leaves, of Betula and called it as dormin. In 1965, flowers and fruits by forming abscission it was found by Cornsforth et al., that both layers. dormin and abscission are chemically ABA plays an important role in plants same compounds and called Abscisic during water stress and during drought Acid (ABA). conditions. It results in loss of turgor 2. Occurrence and closure of stomata. This hormone is found abundantly inside It has anti-auxin and anti-gibberellin the chloroplast of green cells. property. Abscisic acid promotes senescence in 183  leaves by causing loss of chlorophyll 15.3 Plant Movements pigment decreasing the rate of Plants have the capacity for photosynthesis and changing the rate changing their positions of proteins and nucleic acid synthesis. in response to external 8. Agricultural Role or internal stimuli, which In Cannabis sativa, induces male flower are known as plant formation on female plants. movements. Movements are basically of two types: I. Vital movements Induction of flowers in short day plants. and II. Physical movements (hygroscopic) It promotes sprouting in storage organs (Figure 15.14). like Potato. ABA plays an important role in plants I. Vital movements during water stress drought conditions. Vital movements are those which are It inhibits the shoot growth and exhibited by the living cells or plants or promotes growth of root system. This organs and they are always related to character protect the plants from water the irritability of the protoplasm. These stress. Hence, ABA is called as stress movements are of two types: hormone. A. Movements of locomotion B. Movements of curvature Plant Movement Vital Movement Physical Movement Movement of locomotion Movement of curvature Autonomic Paratonic Autonomic Paratonic (Spontaneous) (Tactic/ Induced) (Spontaneous) (Tactic/ Induced) Ciliary Phototactic Amoeboid Chemotactic Cyclosis Thermotactic Tropic Nastic (Growth Movement) (Variation Movement) Geotropic Nyctinastic Phototropic Seismonastic Thigmotropic Thigmonastic Growth Movement Variation Movement Hydrotropic Hyponastic Chemotropic Epinastic Thermotropic Nutational Aerotropic Figure 15.14: Types of Plant Movements 184  Paratonic or Tactic (induced) movement of locomotion Phototactic Chemotactic Thermotactic These movements It occurs in response to It occurs in response occur in response to chemical stimulus. to heat stimulus. light. Example: Antherozoids in Example: Example: Zoospores Bryophytes and Chlamydomonas of Chlamydomonas Pteridophytes are moves from cold to warm water attracted to chemical substances of Archegonia Figure 15.15: Types of Paratonic movements A. Movements of locomotion i. Autonomic movements of curvature These movements include the movement The movement arising from internal of protoplasm inside the cell or movement changes or internal stimuli of plant of whole unicellular or multicellular plant body is called autonomic movement body as in Chlamydomonas, gametes and of curvature. This does not require any zoospores. external stimulus. They are two types: i. Autonomic movements of locomotion a. Autonomic movement of growth: It is of the following types: The movements arising from internal changes or internal stimuli of plant 1. Hyponasty: When growth is more body is called autonomic movements of on lower surface, petals show curvature locomotion. This movement takes place on upper side and ultimately the flower due to the presence of cilia or flagella and becomes closed. Such type of movement movement of cytoplasm (Cyclosis). is called hyponasty. 2. Epinasty: When the growth is more ii. Paratonic or Tactic (induced) on upper surface, petals show curvature movements of locomotion on the lower side and ultimately the The movements due to external factors flower opens. Such movement is called or stimuli like light, temperature and epinasty. The flower usually opens at chemicals are called paratonic movement high temperature and remains closed at of locomotion (Figure 15.15). low temperature (Figure 15.16). B. Movement of curvature 3. Nutation: The growth of the stem In higher plants they are restricted only apices occurs in a zig-zag manner. It is to bending or curvature of some of their because the two sides of the stem apex parts. There are mainly two types: alternatively grow more. Such growth movements are called as nutational They are i) Autonomic movement of movements. In some plants nutational curvature and ii) Paratonic movement of movements allow the shoots apex curvature. to grow in helical path in upward 185  Hyponasty Epinasty Figure 15.16: Hyponasty and Epinasty direction. This movement is called circumnutation. It is commonly found in the stems of climbers of Cucurbitaceae (Figure 15. 17). b. Autonomic Movement of variation: It happens in Indian telegraph plant. (Desmodium gyrans). The compound leaf consists of a larger terminal and two smaller lateral leaflets. During day time, the two lateral leaflets move upward at an angle of 90° and come to lie parallel to the rachis. Again, they may move downward at 180° so that they are Figure 15.17: Circumnutation parallel to the rachis. They may again move upward at 90° to come in their original position. All these movements occur with jerks after intervals, each movement being completed in about 2 minutes (Figure 15.18). ii. Paratonic (induces) movements of curvature The movement arising from external stimulus is called Paratonic (Induced) movements of curvature. They are of two types. 1) Tropic movements 2) Nastic movements (Table 2) a. Tropic movements A movement that occurs in response to Figure 15.18: Autonomic Movement of an unidirectional stimulus is called tropic Variation 186  Table 2: Differences between Tropic Movements and Nastic movements Tropic Movements Nastic movements 1. Movement occurs due to unidirectional These movements occur due to a stimulus. diffused stimulus. 2. The stimulus acts on protoplasm from one The stimulus acts on the protoplasm direction only. from all sides. 3. The response is directly related to the The response has no relation to the direction of the stimulus. direction of the stimulus but with organ. 4. These are movements of curvature caused These are also the movement of by unilateral growth. curvature but they are caused by reversible turgor changes. 5. Tropic movements may be phototropic, Nastic movements may be geotropic, hydrotropic, thigmotropic, seismonastic, photonastic or chemotropic, thermotropic or aerotropic. thermonastic movement or tropism. There are seven flowers move towards the stimulus of light types in tropic movements (Geotropic, and are said to be positively phototropic Phototropic, Thigmotropic, Chemotropic, while others such as roots and rhizoids Hydrotropic, Thermotropic and Aerotropic) which move away from the stimulus of 1. Geotropism light are called negatively phototropic. The movements which take place in b. Nastic Movements response to gravity stimulus are called When growth movements occur in geotropic movements. The primary response to an external stimulus which is roots growing down into soil are not unidirectional but diffused, they are positive geotropic. Primary stems that called nastic or paratonic movements of grow away from soil (against gravity) variation. Paratonic variation movements are negative geotropic. Secondary roots are determined by some external stimuli, growing at right angles to the force of light, temperature, chemicals and touch. gravity are Diageotropic. Secondary They are: lateral roots which grow obliquely downwards are Plagiogeotropic. 1. Nyctinastic movement (or) sleep Lateral roots and branches which are movement not sensitive to gravitational stimulus The diurnal (change in day-night) are Apogeotropic. movements of leaves and flowers of some 2. Phototropism species which take up sleep position at The tropic movement taking place as night are called nyctinastic movements. a response to light stimulus is called They are caused by relative changes in phototropism. Some of the plant parts such cell size on the opposite sides of the leaf as stems, branches, leaves and pedicels of base called pulvinus. The movements 187  are attributed to the amount of auxin and Experiment to demonstrate K1 ions. The entry of water to the lower negative geotropism in aerial stem side of the pulvinus causes the leaves to The Clinostat has a rotating pot like container mounted on an axis rod. stand erect and exit of water causes them A potted plant is fitted horizontally to drop. They are of two types: on the Clinostat and rotated slowly which completely eliminates gravity i. Photonasty as all the sides of the plant are equally The nastic movement caused in response to stimulated. If the rotation of the light is called photonasty or photonastic Clinostat is stopped for a considerable period of time then the tip of the movement. The opening of leaves and stem is observed to curve and grow flowers during daytime and their close at upwards this proves that the stem tip night is an example. is negatively geotropic (Figure 15.19). ii. Thermonasty Clinostat The nastic movement taking place in response to temperature is called thermonasty or thermonastic movement. In Crocus the flowers open at high temperature and close at low temperature. Figure 15.19: Clinostat 2. Seismonastic movement This means a response to shaking. The Experiment to demonstrate positive best example is Mimosa pudica (Touch- phototropism in shoot tips A darkened black box is taken having a small window on one side. A well- watered potted plant is placed inside the box. This is referred to as a phototropic chamber or heliotropic chamber. If the window is kept closed for about 24 hours the plant shows normal growth. If the window is kept opened, it is found after two days that the shoot tip bends and grows towards light proving that it is positively phototropic (Figure 15.20). Figure 15.21: Mimosa pudica showing Closed Window Open Window response to touch me-not plant) which is a sensitive plant Direction of Darkness Light (Figure 15.21). Such plants respond to stimuli such as touch, blow or metallic shock by folding their leaflets and lowering their leaves.This effect is caused Figure15.20: Experiment to demonstrate by the movement of water in and out of Photropism the parenchymatous cells of the pulvinus (Figure 15.22). 188  b c (d) (e) plasma H 2O membrane cell wall flexor cells turgid extensor Ca2+ us cells TnV in O lv H2 pu H+ N turgid state K+ Cl - shrinking swelling + Cl- stretched K flaccid state flexor cells H+ flaccid extensor H 2O H 2O H O 2 O cells 2 H H2 O H 2O TnV TnV N Figure 15.22: Mechanism of Seismonastic movement in Mimosa pudica 3. Thigmonastic movement hairs are activated. Similarly, in dionaea, The movements found in the leaves of the two halves of the leaf curve upwards Drosera and Dionaea (Venus fly trap) result along the midrib. These parts of the leaves in response to the touch stimulus of insects. come to their normal position after the As soon as an insect sits on the leaf the cilia insect has been digested (Figure 15.23). curve inward to trap the insect and trigger II Physical Movement (Hygroscopic Movements) Physical movements are those which are found in dead parts of the plants and they are not related to any irritability of the protoplasm. They are also called hygroscopic movements or mechanical movements. Dispersal of spores and seeds, dehiscence of sporangia, bursting of seeds and movement of elaters are the examples of physical or hygroscopic Figure 15.23: Thigmonasty in Dionaea movement. 189 15.4 Photoperiodism Intermediate plants Day neutral plants Trees take several years for initiation of flowering whereas an annual herb flowers within few months. Each plant requires Photoperiodism in plants a specific time period to complete their vegetative phase which will be followed by reproductive phase as per their Long day plants Short day plants internal control points through Biological Clock. The physiological mechanisms in relation to flowering are controlled by Short long day Long short day (i) light period (Photoperiodism) and plants plants (ii) temperature (Vernalization). The Figure: 15.24 Classification of Plants based physiological change on flowering due on Photoperiodism to relative length of light and darkness iii. Short day plants: The plants that require (photoperiod) is called Photoperiodism. a short critical day length for flowering The term photoperiodism was coined are called short day plants or long night by Garner and Allard (1920) when they plants. Example: Tobacco, Cocklebur, observed this in ‘Biloxi’ variety of soybean Soybean, Rice and Chrysanthemum. (Glycine max) and ‘Maryland mammoth’ variety of tobacco (Nicotiana tabacum). iv. Long short day plants: These are The photoperiod required to induce actually short-day plants but they have flowering is called critical day length. to be exposed to long days during their Maryland mammoth (tobacco variety) early periods of growth for flowering. requires 12 hours of light and cocklebur Example: Some species of Bryophyllum (Xanthium pensylvanicum) requires and Night jasmine. 15.05 hours of light for flowering. v. Intermediate day plants: These require a photoperiod between long day and 1. Classification of plants based on short day for flowering. Example: Photoperiodism Sugarcane and Coleus. Depending upon the photoperiodic vi. Day neutral plants: There are a responses plants are classified as given in number of plants which can flower in Figure 15.24. all possible photoperiods. They are also i. Long day plants: The plants that require called photo neutrals or indeterminate long critical day length for flowering plants. Example: Potato, Rhododendron, are called long day plants or short night Tomato and Cotton. plants. Example: Pea, Barley and Oats. 2. Photoperiodic induction ii. Short long day plants: These are long day plants but should be exposed to An appropriate photoperiod in 24 hours’ short day lengths during early period of cycle constitutes one inductive cycle. growth for flowering. Example: Wheat Plants may require one or more inductive and Rye. cycles for flowering. The phenomenon of 190 conversion of leaf primordia into flower 5. Phytochrome primordia under the influence of suitable X inductive cycles is called photoperiodic Day induction. Example: Xanthium (SDP) – 1 Pr 660nm Pfr Pfr X Physiological response 730nm inductive cycle and Plantago (LDP) – 25 Night inductive cycles. Phytochrome is a bluish biliprotein pigment 3. Site of Photoinductive perception responsible for the perception of light in Photoperiodic stimulus is perceived by the photo physiological process. Butler et al., leaves. Floral hormone is synthesised in (1959) named this pigment and it exists leaves and translocated to the apical tip to in two interconvertible forms: (i) red light promote flowering. This can be explained absorbing pigment which is designated as by a simple experiment on Cocklebur Pr and (ii) far red light absorbing pigment (Xanthium pensylvanicum), a short day which is designated as Pfr. The Pr form plant. Usually Xanthium will flower under absorbs red light in 660nm and changes to Pfr. short day conditions. If the plant is defoliated The Pfr form absorbs far red light in 730nm and kept under short day conditions it will and changes to Pr. The Pr form is biologically not flower. Flowering will occur even when inactive and it is stable whereas Pfr form is all the leaves are removed except one leaf. biologically active and it is very unstable. In If a cocklebur plant is defoliated and kept short day plants, Pr promotes flowering and under long day conditions, it will not flower. Pfr inhibits the flowering whereas in long If one of its leaves is exposed to short day day plants flowering is promoted by Pfr and condition and rest are in long day condition, inhibited by Pr form. Pfr is always associated flowering will occur (Figure 15.25). with hydrophobic area of membrane systems while Pr is found in diffused state in the The nature of flower producing cytoplasm. The interconversion of the two stimulus has been elusive so far. It is forms of phytochrome is mainly involved in believed by many physiologists that it flower induction and also additionally plays is a hormone called florigen. The term a role in seed germination and changes in florigen was coined by Chailakyan membrane conformation. (1936) but it is not possible to isolate. Short Day Long Day 4. Importance of photoperiodism Short 1. The knowledge of photoperiodism Day plays an important role in hybridisation experiments. 2. Photoperiodism is an excellent example of physiological pre-conditioning that is using an external factor to induce A B C D E F physiological changes in the plant. Figure 15.25: Experiment on Cocklebur plant showing photoperiodic stimulus 191 192 15.5 Vernalization (Vernal – Spring Like) C Devernalization Besides photoperiod certain plants require High temperature a low temperature exposure in their B Chilling earlier stages for flowering. Many species Vernalin D Translocation of flower A inducing substance of biennials and perennials are induced to Florigen F flower by low temperature exposure (0oC to Precursor 5oC). This process is called Vernalization. The term Vernalization was first used by T. D. Lysenko (1938). 1. Mechanism of Vernalization: Figure 15.26: Vernalization and Flowering Two main theories to explain the seeds are transferred to low temperature mechanism of vernalization are: (3oC to 5oC) from few days to 30 days. i. Hypothesis of phasic development Germinated seeds after this treatment are ii. Hypothesis of hormonal involvement allowed to dry and then sown. The plants will show quick flowering when compared i. Hypothesis of phasic development to untreated control plants. According to Lysenko, development of an annual seed plant consists of two phases. 3. Devernalization First phase is thermostage, which is Reversal of the effect of vernalization is vegetative phase requiring low temperature called devernalization. and suitable moisture. Next phase is photo 4. Practical applications stage which requires high temperature for 1. Vernalization shortens the vegetative synthesis of florigen (flowering hormone). period and induces the plant to ii. Hypothesis of hormonal involvement flower earlier. According to Purvis (1961), formation 2. It increases the cold resistance of the of a substance A from its precursor, plants. is converted into B after chilling. The 3. It increases the resistance of plants to substance B is unstable. At suitable fungal disease. temperature B is converted into stable 4. Plant breeding can be accelerated. compound D called Vernalin. Vernalin is converted to F (Florigen). Florigen induces 15.6 Seed Germination and Dormancy flower formation. At high temperature B is converted to C and devernalization occurs I. Seed Germination (Figure 15.26). The activation and growth of embryo from seed into seedling during favourable 2. Technique of Vernalization: conditions is called seed germination. The seeds are first soaked in water and allowed to germinate at 10o C to 12o C. Then 193 but some seeds do not germinate when due to blockage by cork cells. These suitable conditions like water, oxygen and seeds are shaken vigorously to remove favourable temperature are not available. the plug which is called Impaction. Germination of such seeds may be delayed iii. Stratification: Seeds of rosaceous for days, months or years. The condition plants (Apple, Plum, Peach and Cherry) of a seed when it fails to germinate even in will not germinate until they have been suitable environmental condition is called exposed to well aerated, moist condition seed dormancy. There are two main under low temperature (0oC to 10oC) reasons for the development of dormancy: for weeks to months. Such treatment is Imposed dormancy and innate dormancy. called Stratification. Imposed dormancy is due to low moisture iv. Alternating temperatures: Germination and low temperature. Innate dormancy is of some seeds is strongly promoted related to the properties of seed itself. by alternating daily temperatures. An alternation of low and high temperature 1. Factors causing dormancy of seeds: improves the germination of seeds. i. Hard, tough seed coat causes barrier v. Light: The dormancy of photoblastic effect as impermeability of water, gas seeds can be broken by exposing them and restriction of the expansion of to red light. embryo prevents seed germination. ii. Many species of seeds produce 15.7 Senescence imperfectly developed embryos called Plant life comprises some sequential events, rudimentary embryos which promotes viz: germination, juvenile stage, maturation, dormancy. old age and death. Old age is called iii. Lack of specific light requirement leads senescence in plants. Senescence refers to to seed dormancy. all collective, progressive and deteriorative iv. A range of temperatures either higher processes which ultimately lead to complete or lower cause dormancy. loss of organization and function. Unlike v. The presence of inhibitors like phenolic animals, plants continuously form new compounds which inhibits seed organs and older organs undergo a highly germination cause dormancy. regulated senescence program to maximize nutrient export. 2. Methods of breaking dormancy: The dormancy of seeds can be broken by 1. Types of Senescence different methods. These are: Leopold (1961) has recognised four types i. Scarification: Mechanical and chemical of senescence: treatments like cutting or chipping of i. Overall senescence hard tough seed coat and use of organic ii. Top senescence solvents to remove waxy or fatty iii. Deciduous senescence compounds are called as Scarification. iv. Progressive senescence ii. Impaction: In some seeds water and oxygen are unable to penetrate micropyle 195  secretes hydrolytic enzymes. The branch of botany which deals with The starch content is decreased in the ageing, abscission and senescence is cells. called Phytogerontology Photosynthesis is reduced due to loss of i. Overall senescence: This kind of chlorophyll accompanied by synthesis senescence occurs in annual plants and accumulation of anthocyanin when entire plant gets affected and dies. pigments, therefore the leaf becomes red. Example: Wheat and Soybean. It also There is a marked decrease in protein occurs in few perennials also. Example: content in the senescing organ. Agave and Bamboo. RNA content of the leaf particularly ii. Top senescence: It occurs in aerial parts rRNA level is decreased in the cells of plants. It is common in perennials, due to increased activity of the enzyme underground and root system remains RNAase. viable. Example: Banana and Gladiolus. DNA molecules in senescencing leaves iii. Deciduous senescence: It is common degenerate by the increased activity of in deciduous plants and occurs only in enzyme DNAase. leaves of plants, bulk of the stem and 3. Factors affecting Senescence: root system remains alive. Example: ABA and ethylene accelerate senescence Elm and Maple. while auxin and cytokinin retard iv. Progressive senescence: This kind of senescence. senescence is gradual. First it occurs Nitrogen deficiency increases in old leaves followed by new leaves senescence whereas nitrogen supply then stem and finally root system. It is retards senescence. common in annuals (Figure 15.28). High temperature accelerates senescence 2. Physiology of Senescence but low temperature retards senescence. Cells undergo changes in structure. Senescence is rapid in dark than in Vacuole of the cell acts as lysosome and light. Overall senescence Top senescence Deciduous senescence Progressive senescence Figure 15.28: Different types of senescence in plants 196  Water stress leads to accumulation of its vascular system to prevent loss of water ABA leading to senescence. and nutrients. Final stage of senescence is abscission. In temperate regions all the 4. Programmed cell death (PCD) leaves of deciduous plants fall in autumn Senescence is controlled by plants own and give rise to naked appearance, then the genetic programme and death of the plant new leaves are developed in the subsequent or plant part consequent to senescence is spring season. But in evergreen plants there called Programmed Cell Death. In short is gradual abscission of leaves, the older senescence of an individual cell is called PCD. leaves fall while new leaves are developed The proteolytic enzymes involving PCD continuously throughout the year. in plants are phytaspases and in animals are caspases. The nutrients and other 6. Morphological and Anatomical substrates from senescing cells and tissues changes during abscission are remobilized and reallocated to other parts Leaf abscission takes place at the base of of the plant that survives. The protoplasts petiole which is marked internally by a of developing xylem vessels and tracheids distinct zone of few layers of thin walled die and disappear at maturity to make them cells arranged transversely. This zone is functionally efficient to conduct water for called abscission zone or abscission layer. transport. In aquatic plants, aerenchyma An abscission layer is greenish-grey in is normally formed in different parts of the colour and is formed by rows of cells of 2 to plant such as roots and stems which encloses large air spaces that are created through PCD. 15 cells thick. The cells of abscission layer In the development of unisexual flowers, separate due to dissolution of middle lamella male and female flowers are present in earlier and primary wall of cells by the activity of stages, but only one of these two completes enzymes pectinase and cellulase resulting its development while other aborts through in loosening of cells. Tyloses are also formed PCD (Figure 15.29). blocking the conducting vessels. Degrading of chlorophyll occur leading to the change 5. Abscission in the colour of leaves, leaf detachment Abscission is a physiological process of from the plant and leaf fall. After abscission, shedding of organs like leaves, flowers, fruits outer layer of cells becomes suberized by the and seeds from the parent plant body. When development of periderm (Figure 15.30). these parts are removed the plant seals off Mitochondria Vacuole Nucleus Plastid Figure 15.29: Programmed cell death 197  functioning of plants under adverse Pholoem environmental conditions is called stress Xylem physiology. Jacob Levitt (1972) first used the term biological stress in relation to plants and according to him stress is “any change in environmental condition that might adversely change the growth and Cortex development of a plant”. The reaction of plants facing stress is called Abscission layer strain. For example, if a normal plant growing under favourable light conditions is subjected to low light intensity, its photosynthesis is reduced. Thus, low light intensity is referred as stress and reduction of photosynthesis is referred as strain. Biological strains are of Figure 15.30: L.S of petiolar base showing two types; Elastic biological strain and Plastic abscission layer biological strain. If the reaction of plant function is temporary and when it returns to 7. Hormones influencing abscission its original state it is called elastic biological All naturally occurring hormones strain. Example: Temporary wilting. If influence the process of abscission. Auxins the reaction is permanent and the plant and cytokinins retard abscission, while function does not return to the normal state abscisic acid (ABA) and ethylene induce it. it is called plastic biological strain. Example: Permanent wilting. Some plants get adapted 8. Significance of abscission to stress condition and are not adversely 1. Abscission separates dead parts of the affected by stress. Such plants are called plant, like old leaves and ripe fruits. stress resistant or stress tolerant plants. 2. It helps in dispersal of fruits and Example: Mangroves. Some plants cannot continuing the life cycle of the plant. face stress and they pass their adverse period 3. Abscission of leaves in deciduous in dormant state and so they are called stress plants helps in water conservation enduring plants. Ephemeral plants are short during summer. lived desert plants, which complete their life 4. In lower plants, shedding of vegetative cycle during the seasonal rains before the parts like gemmae or plantlets help in onset of dry season. These ephemeral plants vegetative reproduction. are called stress escapers. Stress in plants can be classified as given in figure 15.31. 15.8 Stress Physiology 1. Biotic Stresses Like all other organisms, plants are also These are adverse effects on plants caused subjected to various environmental stresses by other living organisms such as viruses, such as water deficit, drought, cold, heat, bacteria, fungi, parasites, insects, weeds and salinity and air pollution. The study of 198