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Growth is affected by the factor which affect the activity of protoplasm. It is affected by a large number of factors both environmental and physiological. Physiologogical factors such as absorption of water, minerals, photosynthesis, respiration etc, and environmental factors including climatic and edaphic both. The effect on these factors on one region of plant are also transmitted to other region of the plant.
Since growth is a resultant of many metabolic processes, it is affected by many external and internal factors, which are as follows,
(i) External factors
(a) Light : Light affects variously e.g., light intensity, quality and periodicity.
❒ Intensity of light : In general, light retards growth in plants. High light intensities induce dwarfing of the plant. Plants at hill tops are short whereas those of a valley are quite tall. Very weak light induces the rate of overall growth and also photosynthesis. Development of chlorophyll is dependent on light and in its absence etiolin compounds in formed which gives yellow colour to the plant. The phenomenon is called etiolation. Similarly high light intensity affecting indirectly increases the rate of water lose and reduces the rate of water growth.
❒ Quality of light : The different colours (different wavelengths) affect the growth of plant. In blue-violet colour light internodal growth is pronounced while green colour light reduces the expansion of leaves as compared to complete spectrum of visible light. The red colour light favours elongation but they resemble etiolated plants. Infrared and ultraviolet are detrimental to growth. However, ultraviolet rays are necessary for the development of anthocyanin pigments in the flowers. Blue and violet colours increase size of lamina of leaf.
❒ Duration of light : There is remarkable effect on durtion of light on the growth of vegetative as well as reproductive structures. The induction and suppresion of flowering are dependent on duration. The phenomenon is termed photoperiodism.
(b) Temperature : Temperature has pronounced effect on the growth of plant. The temperature cardinals for growth vary according to temperature zones. The minimum, optimum and maximum temperatures are usually 5oC (arctic), 20 – 30oC (temperate) and 35 – 40oC (tropical). The optimum temperature needed for the growth of a plant is much dependent on the stages of development. Low temperatures during nights reduces the rate of respiration and high temperature during days increases photosynthesis accumulated photosynthate also increases growth the tomato plants do not grow well under uniform temperatures condition of day and night but they grow well under low night temperature (nyctotemperature) and flucuating day temperature (phototemperature). This response of plant to temperature variation is called thermoperiodicity. When plants are exposed to extremes of temperature they get injured and the injuries are called descication, chilling and freezing.
Due to hot or cold spells of wind, when the transpiration exceeds absorption, the plant tissue gets injured and the injury is called desiccation. If a plant of hot climate is exposed to low temperature it gets injured and the injury is called chilling. During winter, in hill plants water is withdrawn from the cell into the intercellular space. As a result, the dehydrated protoplasm coagulates. There is inter and intracellular ice formation due to further lowering of temperature and as a result the plant tissue is injured. This injury is called freezing. A plant develops high osmotic concentration of the cell sap and a thick bark to withstand these injuries. Besides, it also shows formation of seeds, spores, tubers etc. when the temperature goes down.
(c) Water : As water is an essential constituent of the living cell, a deficiency of water causes stunted growth. Moreover unless the cells are in a turgid condition, they cannot divide and unless new cells are added up by the activity of the meristems, growth cannot take place. water is also essential for photosynthesis not only as a raw material, but also for the photosynthetic activity of the cells. Water is also essential for the translocation of mineral salts and ready-made food to the growing regions of the plants. Without food supply growth cannot take place.
(d) Oxygen : In poorly aerated soil there is low concentration of oxygen and a high concentration of Under such conditions plants usually show stunted growth. Normal growth of most plants occurs only when abundant oxygen is present since is important for respiration. It has been reported that oxygen plays some important role during GI stage of cell division.
(e) Mineral salt : Absence of essential mineral salts results in abnormal growth. For example, the absence of nitrogen prevents protein-synthesis, while the absence of iron prevents chlorophyll formation and thus leads to pale and sickly growth of plants, known as chlorotic condition.
(f) Pollutants : Several pollutants such as automobile exhaust, peroxyacetyl nitrate (PAN), pesticites etc have detrimental effect on plant growth. Some plants are very sensitive to certain pollutants. Citrus and Gladiolus are very sensitive to fluorides. Poor growth of tobacco is observed in regions where ozone concentration is high. White pine cannot survive under high concentration. Cotton plants are, similarly very sensitive to ethylene.
(g) Carbon dioxide : CO2 is essential for photosynthesis and hence nutrition. Due to change in photosynthetic rate, with the increase or decrease in concentration, the plant growth is also affected.
(ii) Internal factors : Amongst internal factor i.e., age, health, hereditory factors, growth regulator, nutritional relations, etc. growth regulators are very important. Some of the internal factors are :
(a) Nutrition : It provides raw material for growth and differentiation as well as source of energy. C/N (carbohydrate/protein) ratio determines the type of growth. High C/N ratio stimulates wall thickening. Less protoplasm is formed. Low C/N ratio favours more protoplasm producing thin walled soft cells. According to law of mass growth, the initial rate of growth depends upon the size of germinating structure (seed, tubes, rhizome, bulb, etc.)
(b) Growth regulators : These are manufactured by living protoplasm and are important internal growth regulators which are essential for growth and development. These growth regulators include several phytohormones and some synthetic substances.
The term hormone used by first Starling (1906). He called it stimulatory substance. The growth and development in plants is controlled by a special class of chemical substances called hormones. These chemicals are synthesized in one part of the plant body and translocated to another where they act in a specific manner. They regulate growth, differentiation and development by promoting or inhibiting the same. They are needed in small quantities at very low concentrations as compared to enzyme. They are rarely effective at the site of their synthesis.
Thus, growth hormones also called phytohormones term given by Thimann (1948), it can be defined as ‘the organic substances which are synthesized in minute quantities in one part of the plant body and transported to another part where they influence specific physiological processes’. Sometimes the term growth regulators is misled with phytohormones. The term phytohormones as the definition indicates, is implied to those chemical substances which are synthesized by plants and thus, they are naturally occurring. On the other hand, there are several manufactured chemicals which often resemble the hormones in physiological action and even molecular structure. Thus the synthetic substances which resemble with hormones in their physiological action are termed as growth regulators.
Phytohormones can have a promoting or inhibiting effect on a process. A particular hormone may promote certain processes, inhibit some others and not effect many others. In general, developmental processes are controlled by more than one growth regulator. They may act synergistically i.e., in a cooperative and beneficial manner (e.g., morphogenesis by auxins and cytokinins) or antagonistically i.e., in opposite manner (e.g., seed germination is promoted by gibberallin and is inhibited by abscisic acid). A group of plant hormones including auxins, gibberellins, cytokinins, ethylene and abscisic acid are presently known to regulate growth.
❒ ” An organic compound characterized by its capacity in low concentration (below 10–3 m or 0.001 m) to induce elongation in shoot cells and inhibition of elongation of root cells.
DISCOVERY AND HISTORICAL ASPECTS
❒” Julius and Sachs (1790) indicated the presence of organ forming substances in plants for the first time.
❒ ” Charles Darwin (1881) discovered the direction of growth of the coleoptile in Phalaris canariensis and observed that when the tip of coleoptile exposed to light, the direction of growth got influenced. He also observed that when the coleoptile is illuminated from one side, it bents towards light but no bending response was observed when the lower part was covered with metal.
❒ ” Darwin concluded that some kind of influence is generated at the tip which is then transmitted to the base.
❒ ” Boyson-Jensen (1910-1913) observed that coleoptile with decapitated tips neither grow nor bend towards light but when the decapitated tip is replaced, curvature towards light takes place. He concluded that the influence was due to chemical diffusing through the block of gelatin.
❒ ” Paal (1914-19) demonstrated that coleoptiles would bend even in dark after certain treatment.
❒ ” F. W. Went (1928) observed that the coleoptile grow and bend away from the side on which the agar block was placed and also stated that the degree of curvature of the coleoptile was directly proportional to the concentration of the chemical influence in the agar block.
❒ ” Went coined the term auxin (Greek Auxein: to grow) to the chemical influence responsible for the phototropic response.
❒ ” Kogl and Haggen Smith (1931) isolated substances form human urine, i.e., Auxin-A and then in 1934, Auxin-B from corn germ oil. Re-examination of human urine led to discovery of another substance called heteroauxin and it was similar to IAA (Indole acetic acid).
❒ ” Thimann (1936) isolated IAA from Rhizopus suinus and then from Avena coleoptile and other plants.
❒ ” IAA is a true natural auxin of plants. Auxins were first of all discovered from human urine.
(ii) Types of auxins : There are two major categories of auxins natural auxins and synthetic auxins.
(a) Natural auxins : These are naturally occurring auxins in plants and therefore, regarded as phytohormones. Indole 3-acetic acid (IAA) is the best known and universal auxin. It is found in all plants and fungi.
The first naturally occuring auxin was isolated by Kogl and Haagen-Smit (1913) from human urine. It was identified as auxin-a (auxentriolic acid, C18H32O5). Later, in 1934 Kogl, Haagen-Smit and Erxleben obtained another, auxin, called auxin-b (auxenolonic acid, C18H30O4) from corn germ oil (extracted from germinating corn seeds), and heteroauxin from human urine. Heteroauxin (C10H9O2N) also known as indole-3-acetic acid (IAA), is the best known natural auxin, Besides IAA, indole-3-acetaldehyde, indole-3-pyruvic acid, indole ethanol, 4-chloro-idole actic acid (4-chloro-IAA) etc., are some other natural auxins.
Natural auxins are synthesized (Young) in physiologically active parts of plants such as shoot apices, leaf primordia and developing seeds, buds (apex), embryos, from amino acid tryptophan. In root apices, they are synthesized in relatively very small amount. Auxins show polar movement. It is basipetal (from apex to base) in stem but acropetal (from root tip towards shoot) in the root. Auxins move slowly by diffusion from cell to cell and not through the vascular tissues. Auxins help in the elongation of both roots and shoots. However, the optimum concentration for the two is quite different.
It is 10 ppm for stem and 0.0001 ppm for the root. Its translocation rate is 1–1.6 cm/hr. (In roots 0.1 to 0.2 cm/hr). Higher concentration of auxins show inhibitory effect on growth.
Natural auxins are of two types : free and bond auxins. The auxins which can easily be extracted are called free auxins, whereas auxins which are hard to extract and need the use of organic solvents are termed as bound auxins. The free form of auxin is active, while the bound auxin is inactive in growth. A dynamic equilibrium exists between these two forms.
(b) Synthetic auxins : These are synthetic compounds which cause various physiological responses common to IAA. Some of the important synthetic auxins are 2, 4-D (2, 4-dichlorophenoxy acetic acid) is the weedicide, 2, 4, 5-T (2, 4, 5-trichlorophenoxy acetic acid), IBA (indole 3-butyric acid), NAA (naphthalene acetic acid, PAA (Phenyl acetic acid), IPA (Indole 3-propionic acid). IBA is both natural and synthetic auxin. Certain compounds inhibit action of auxin and compete with auxins for active sites are called antiauxins. e.g., PCIB (p- chlorophenoxy isobutyric acid), TIBA (2, 3, 5-tri iodobenzoic acid). TIBA is used in picking cotton bolls.
(iii) Bioassay of Auxins : Testing of biological activity (growth) of a substance (auxin) by employing living material is called bioassay. Auxin bioassay is also quantitative test as it measures amount of effect in response to a particular concentration of auxin.
(a) Avena coleoptile curvature test : Avena curvature test carried out by F.W. Went (1928), demonstrated the effect of auxins on plant growth by performing some experiments with the oat (Avena sativa) coleoptile.
❒ When the tips of the coleoptiles were removed, no growth took place.
❒ When the freshly cut coleoptiles were placed on agar blocks for a few hours (during this period auxin diffused into the agar block) and then the agar blocks were placed on the cut ends of the coleoptile, growth occurred.
❒ When the agar block with the diffused substance was placed laterally on the cut tip of the coleoptile, only that side of the coleoptile elongated resulting in a curvature.
(b) Split pea stem curvature test : This test was also discovered by Went, 1934. Dark germinated seeds of pea are decapitated. About half an inch part of stem between 2nd and 3rd node is removed and split longitudinally. It is then floated on the test solution contained in a beaker. At first, negative curvature occurs due to water uptake. Then positive curvature occurs which is proportional to the log of the concentration of auxin.
These experiments indicated that some substance is synthesised in the coleoptile tip is translocated downward. He called this substance auxin.
(c) Root growth inhibition test (Cress root inhibition test) : Sterilized seeds of cress are germinated over moist filter paper. Root lengths are measured. 50% of seedlings are placed in test solution while the rest are allowed to grow over the moist filter paper. Lengths of roots are measured after 48 hours. Seedlings placed in test solution show very little root growth while the roots of controlled seedlings show normal growth. The degree of root growth inhibition is proportional to auxin concentration.
(iv) Functions of auxins : Auxins control several kinds of plant growth processes. These are as follows :
(a) Cell elongation : Auxins promote elongations and growth of stems and roots and enlargement of many fruits by stimulating elongation of cells in all directions.
The auxins cause cell enlargement by solubilisation of carbohydrates, loosening of microfibrils, synthesis of more wall materials, increased membrane permeability and respiration.
(b) Apical dominance : In many plants, the apical bud grows and the lower axillary buds are suppressed. Removal of apical bud results in the growth of lower buds. The auxin (IAA) of the terminal bud inhibits the growth of lateral buds. This phenomenon is known as apical dominance.
This property of auxins has found use in agriculture. Sprouting of lateral buds (eyes) of the potato tuber is checked by applying synthetic auxin (NAA).
(c) Control of abscission layer : Auxin inhibits abscission of leaves and fruits. Abscission layer is produced when the auxin content falls below a minimum. Addicot and Lynch (1951) put forward auxin gradient theory about abscission :
❒ No abscission if auxin content is high on the organ side.
❒ Abscission layer begins formation when auxin content becomes same on stem and organ sides.
❒ Abscission is favoured when auxin content is low on the organ side.
Premature drop of fruits such as apple, pear and citrus can be prevented to a great extent by spraying the trees with a dilute solution of IAA, NAA or some other auxin.
(d) Weed control : Weeds are undesirable in a field with a crop. Weeds cause competition for water, mineral, light and space. This causes poor yield. By the spray of 2, 4-D, broad-leaved weeds can be destroyed but 2, 4-D does not affect mature monocotyledonous plants.
(e) Root differentiation : Many new plants are usually propogated by stem cutting e.g., Rose, Bougainvillea. If we dip the lower cut end of a cutting in dilute solution of auxins (specially IBA gives very good results) very soon large number of roots are developed on the cut ends due to which these cuttings develop into successful plants.
(f) Parthenocarpy : It is the process of formation of fruits without fertilization. Such fruits are called as parthenocarpic fruits and are without seeds. Parthenocarpy can be induced by application of IAA in a paste form to the stigma of a flower or by spraying the flowers with a dilute solution of IAA. Banana, oranges and grapes are now-a-days grown parthenocarpically on commercial scale.
(g) Control of lodging : In some plants when the crop is ripe and there is heavy rain accompanied by strong winds, the plants bends as a result of which the ear (inflorescence) gets submerged in water and decays. If a dilute solution of any auxin is sprayed upon young plants the possibility of bending of plants is reduced as the stem becomes stronger by the application of auxins.
(h) Flowering : In pineapple, NAA promotes flowering. In lettuce, auxins help in delaying the flowering. In cotton plants, the use of auxins increases the cotton seeds production.
(i) Differentiation of vascular tissues : Auxins induce the differentiation of xylem and phloem in intact plants and also in callus produced in vitro during tissue culture experiments.
(j) Sex expression : The spray of auxins increases the number of female flowers in cucurbits. In maize application of NAA during the period of inflorescence differentiation can induce formation of hermaphrodite or female flowers in a male inflorescence.
Thus auxins cause femaleness in plants.
(k) Healing : Healing of injury is effected through auxin induced division in the cells around the injured area. The chemical was formerly named traumatic acid or traumatin.
(l) Nodule formation : In legumes, IAA is known to stimulate nodule formation.
(m) Respiration : According to French and Beevers (1953) the auxin may increase the rate of respiration indirectly through increased supply of ADP by rapidly utilizing the ATP in the expanding cells.
Gibberellins are weakly acidic hormones having gibbane ring structure which cause cell elongation of intact plants in general and increased internodal length of genetically dwarfed plants (i.e., corn, pea) in particular.
(i) Discovery : Gibberellins were first isolated from the fungus Gibberella fujikuroi (Fusarium moniliforme) the causal organism of Bakanae disease or foolish seedling disease of rice plants in Japan by Kurosawa in 1926. The characteristic symptoms of this disease are abnormal growth of stem and leaves, thin plants with long internodes, early flowering or death before flowering and fruiting.
In 1939, Yabuta and Sumiki and coworkers working in Tokyo isolated an active substance from the fungus and called it Gibberellin A. This gibberellin preparation was probably a mixture of several gibberellins. The first gibberellin to be obtained was Gibberellin A-3. Cross et al. (1961) explained the detailed structure of gibberellic acid. Now 60 gibberellins have been identified from different groups of plants (algae, fungi, mosses, ferns, gymnosperms and angiosperms).
Many of them occur naturally in plants. Gibberella Fujikuroi has as many as 15 gibberellins. A single plant also possesses a number of gibberellins. All the different types of gibberellins, known so far, have gibbane skeleton and are acidic in nature. Therefore, these are termed as GA1 , GA2 , GA3, GA4 and so on. Of these gibberellic acid or gibberellin is the commonest. Gibberellins are synthesised in plants in leaves of buds, developing embryos, root tips, young apical leaves, shoot tips and seeds. Gibberellins are transported readily in the plant, apparently moving passively in the stream either in xylem or phloem. Their transport in non-polar. Anti-gibberellins like malic hydrazide, phosphon D, Alar and chorocholine cheoride (CCC) or cycocel are also called antiretardants (stimulates flowering and inhibits the growth of nodes). Commercial production of GA is still carried out by culturing this fungus in large vats.
(ii) Mechanism of action : Gibberellins are closely related with steroids. Gibberellins exhibit ecdysome like effects. Ecdysome is a moulting hormone. The steroids have very specific effect in depressing genes and thus activating specific genes. Another significant gibberellin treatment is production of enzymes like amylase and protease. It is also considered that the effect of gibberellin is indirect.
According to this view gibberellins show its physiological effects by altering the auxin status of the tissue.
(iii) Bioassay of gibberellin : Gibberellin bioassay is performed through dwarf maize/pea test and cereal endosperm test.
(a) Dwarf pea bioassay : Seeds of dwarf pea are allowed to germinate till the just emergence of plumule. GA solution is applied to some seedlings others are kept as control. After 5 days, epicotyl length is measured. Increase in length of epicotyl over control seedlings is proportional to GA concentration.
(b) Barley endosperm bioassay : Endosperms are detached from embryos, sterilized and allow to remain in 1ml of test solution for 1-2 days. There is build up of reducing sugars which is proportional to GA concentrations. Reducing sugars do not occur in edoperms kept as control.
(iv) Functions of gibberellin
(a) Stem elongation : The gibberellins induce elongation of the internodes. The cell growth is promoted by the increase in the hydrolysis of polysaccharides. It also increases the elasticity of cell wall. The elongation of stem results due to rapid cell division and cell elongation induced by gibberellins.
(b) Leaf expansion : In many plants leaves become broader and elongated when treated with gibberellic acid. This leads to increase in photosynthetic area which finally increases the height of the plant. Interestingly, gibberellins show no effect on roots.
(c) Reversal of dwarfism : One of the most striking effects of gibberellins is the elongation of genetic dwarf (mutant) varieties of plants like corn and pea. It is believed that dwarfism in the mutant variety of plant is due to blocking of the capacity for normal gibberellin production (deficiency of gibberellin). When gibberellin is applied to single gene dwarf mutants e.g., Pisum sativam, Vicia faba and Phaseolus multiflorus, they grow to their nomal heights. It is further interesting to note that application of gibberellins to normal plants fail to show any remarkable effects.
(d) Bolting and Flowering : Gibberellins induce stem elongation in ‘rosette plants’ e.g., cabbage, henbane, etc. Such plants show retarded internodal growth and profuse leaf development. In these plants just prior to the reproductive phase, the internodes elongate enormously causing a marked increase in stem height. This is called bolting.
Bolting needs long days or cold nights. It has been further noticed that if cabbage head is kept under warm nights, it remains vegetative. The exogenous application of gibberellins induced bolting in first year itself in plants like cabbage (normally bolting occurs next year due to effect of endogenous gibberellins).
(e) Enzyme formation : One of the most dramatic effects of GA is its induction of hydrolytic enzymes in the aleurone layer of endosperm of germinating barley seeds and cereal grains. GA stimulates the production of digestive enzymes like proteases, α-amylases, lipases which help to mobilise stored nutrients. GA treatment stimulates a substantial synthesis of new mRNA. Thus GA acts to uncover or depress specific genes, which then cause the synthesis of these enzymes. It is assumed that GA acts on the DNA of the nucleus.
(f) Breaking of dormancy : Gibberellins overcome the natural dormancy of buds, tubers, seeds, etc. and allow then to grow. In this function gibberellins act antagonistically to abscisic acid (ABA).
(g) Parthenocarpy : Gibberellins have been considered to be more effective than auxins for inducing parthenocarpy in fruits like apple, tomato and pear. GA application has also resulted in the production of large fruits and bunch length in seedless grapes.
(h) Sex expression : Gibberellins control sex expression in certain plants. In general, gibberellin promote the formation of male flowers either in place of female flowers in monoecious plants such as cucurbits or in genetically female plants like Cannabis, Cucumis.
(i) Substitution for vernalization : Vernalization is the low temperature requirement of certain plant (i.e., biennials) to induce flowering. The low temperature requirement of biennials for flowering can be replaced by gibberellins.
(j) Malt yield : There is increased malt production when gibberellins are provided to germinating barley grains (due to greater production of α-amylase).
(k) Delayed ripening : Ripening of citrus fruits can be delayed with the help of gibberellins. It is useful in safe and prolonged storage of fruits.
(l) Seed germination : Gibberellins induce germination of positively photo-blastic seeds of lettuce and tobacco in complete darkness.
(3)Cytokinins (Phytokinins) : Cytokinins are plant growth hormones which are basic in nature, either aminopurine or phenyl urea derivatives that promote cell division (cytokinesis) either alone or in conjugation with auxin.
(i) Discovery : The first cytokinin was discovered by Miller, Skoog and Strong (1955) during callus tissue culture of Nicotiana tobaccum (tobacco).
It was synthetic product formed by autoclaving Herring sperm (fish sperm) DNA. This synthetic product was identified as 6-furfuryl amino-purine and named as kinetin. He found that normal cell division induced by adding yeast extract.
Various terms such as kinetenoid (Burstran, 1961), phytokinin (Dendolph et al. 1963) phytocytomine (Pilet 1965) have been used for kinetin like substances but the term cytokinin proposed by Letham (1963) has been widely accepted. Letham et al. (1964) discovered first natural, cytokinin in unripe maize grain (Zea mays). It was named as zeatin (6 hydroxy 3 methyl trans 2-butenyl amino purine).
About 18 cytokinins have been discovered, e.g., dihydrozeatin, IPA (Isopentenyl adenine), benzyl adenine. The most widely occurring cytokinin in plant is IPA. It has been isolated from Pseudomonas tumefaciens. Many are found as constituents of tRNAs. Cytokinins are synthesized in roots as well as endosperm of seeds. Coconut millk and Apple fruit extract are rich in cytokinins. Cytokinins in coconut milk called coconut milk factor.
Kinetin (6 furfuryl amino purine) is a derivative of the nitrogen base adenine. Plant physiologists use the term cytokinins to designate group of substances that stimulate cell division in plants. Cytokinins are prdouced in actively growing tissues such as embryos, developing fruits and roots. Kinetin is the derivative of purine base adenine, which bears furfuryl group at 9 position which migrated to 6 position of the adenine ring during autoclaving of DNA. According to Fox (1969) cytokinins are substances composed of one hydrophilic group of high specificity (adenine) and one lipophilic group without specificity.
Cytokinin is transported to different parts of the plant through xylem elements. According to Osborne and Black (1964), the movement of cytokinin is polar and basipetal.
(ii) Mechanism of action : Most known cytokinins have an adenine nucleus with purine ring intact with N6 substituents of moderate size. Cytokinins never act alone. In conjugation with auxins, they stimulate cell division even in permanent cells. It was noticed by Skoog and Miller that callus cultures grew slowly on basal medium, but growth could be promoted by adding hormones like IAA and cytokinins. No response occurred with auxin or cytokinin alone. When both the hormones are present in equal amount, cells divide rapidly but fail to differentiate. However, when quantity of cytokinins is more than auxins, shoot bud appears from callus. With more concentration of auxins, roots develop fast. The similarity in structure of most cytokinins to adenine, a constituent of DNA and RNA suggests that basic effect of cytokinin might be at the level of protein synthesis.
(iii) Bioassay of cytokinins : Bioassay is done through retention of chlorophyll by leaf discs, gains of weight of a tissue in culture, excised radish cotyledon expansion, etc.,
(a) Tobacco pith culture : Tobacco pith culture is divided into two weighted lots one supplied with cytokinin and the other without it. After 3-5 weeks, increase of fresh weight of treated tissue over control is noted. It is a measure of stimulation of cell division and hence cytokinin activity.
(b) Retardation of leaf senescence : Leaves are cut into equal sized discs with the help of a cutter. They are devided into two lots. One lot is provided with cytokinin. After 48-72 hours, leaf discs are compared for chlorophyll contents. Cytokinin retards chlorophyll degradation.
(c) Excised radish cotyledon expansion : Excised radish cotyledons are measured and placed in test solution as well as ordinary water (as control). Enlargement of cotyledons indicates cytokinin activity.
(d) Root inhibition test : Kiraly and his coworkers (1966) used root inhibition test for cytokinin bioassay. They found, that amount of root inhibition of actively growing seedlings is related to cytokinin activity.
(iv) Functions of cytokinins
(a) Cell division : Cytokinins are essential for cytokinesis and thus promote cell division. In presence of auxin, cytokinins stimulate cell division even in non-meristematic tissues. In tissue cultures, cell division of callus (undifferentiated mass of parenchyma tissue) is enhanced when both auxin and cytokinin are present. But no response occurs with auxin or cytokinin alone.
(b) Cell enlargement and Differentiation : Under some conditions cytokinins enhance the expansion of leaf cells in leaf discs and cotyledons. These cells considered to be mature and under normal conditions do not expand. Cytokinins play a vital role in morphogenesis and differentiation in plants. It is now known that kinetin-auxin interaction control the morphogenetic differentiation of shoot and root meristems.
(c) Delay in senescence : Cytokinin delay the senescence (ageing) of leaves and other organs by controlling protein synthesis and mobilization of resources (Disappearance of chlorophyll). It is called Richmond Lang effect. It was reported by Richmond and Lang (1957) while working on detached leaves of Xanthium.
(d) Counteraction of apical dominance : Auxins and cytokinins act antagonistically in the control of apical dominance. Auxins are responsible for stimulating growth of apical bud. On the other hand, cytokinins promote the growth of lateral buds. Thus exogenous application of cytokinin has been found to counteract the usual dominance of apical buds.
(e) Breaking of dormancy : Cytokinins breaks seeds dormancy of various types and thus help in their germination. They also induce germination of positively photoplastic seed like lettuce and tobacco even in darkness.
(f) Accumulation and Translocation of solutes : Cytokinins induce accumulation of salts inside the cells. They also help solute translocation in phloem.
(g) Sex expression : Cytokinins promote formation of female flowers in some plants.
(h) Enzyme activity : Cytokinins stimulate the activity of enzymes especially those concerned with photosynthesis.
(i) Parthenocarpy : Development of parthenocarpic fruits through cytokinin treatment has been reported by Crane (1965).
(j) Pomalin : A combination of cytokinin (6-benzladenine) and gibberellin (GA4, GA7) called pomalin is particularly effective in increasing apple size.
(k) Initiation of interfasicular cambium : Cytokinins induce the formation of interfasicular cambium in plants e.g., Pinus radiata.
(l) Nucleic acid metabolism : Guttman (1957) found a quick increase in the amount of RNA in the nuclei of onion root after kinetin treatment.
(m) Protein synthesis : Osborne (1962) demonstrated the increased rate of protein synthesis on kinetin treatment.
(n) Flowering : Gibberellins also play an important role in the initiation of flowering. Lang (1960) demonstrated that added gibberellin could substitute for the proper environmental conditions in Hyoscyamus niger which requires long day treatment for flowering. Such effects of gibberellin are common among vernalised and long day plants.
pe. growth inhibitors
ABscISIc ACID (ABA)
❒” Work on ABA started in laboratory of Dr. Carns (1950) in U.S.A. on cotton plants. Carns and Addicott (1961-65) discovered 2 substances from cotton i.e., Abscisin I from old cotton and Abscisin II from young cotton bolls, both of these are capable of inducing abscission of leaves.
❒” Eagles and Wareing (1963) discovered another substance ‘Dormin’ from dormant buds of Acer plants, capable of causing dormancy of seeds.
❒” Later on Abscisin I, Abscisin II and Dormin were found to be same chemically and common name Absicisic acid (ABA) was given.
❒” It is chemically related to the cytokinins. It is probably universally distributed in higher plants. it is a naturally occurring hormone which inhibits or retards many physiological activities of plants.
❒ ABA is a C-15 sesquiterpene, a compound of three isoprene units. The biosynthesis of ABA probably takes place through mevalonic acid pathway. Most of this hormone is synthesized in leaves and fruits. BIOSYNTHESIS OF ABA
❒ Through mevalonate. The accepted site of ABA production in mature green leaves and fruits. Plastids may be the centres of ABA synthesis.
❒ The ABA is manufactured in mature leaves and translocated to shoot apex. It is transported through both xylem and phloem elements as also through the parenchyma cells out side the bundle.
MODE OF ACTION
❒ The ABA is thought to act in the following manner:
❒ By competing with other hormones for specific enzyme site,
❒ By inhibiting biosynthesis of other hormones,
❒ By inhibiting RNA synthesis (transcription),
❒ By inhibiting protein synthesis (translation).
❒ ” It causes ageing and abscission of leaves. During this process the production of ethylene is promoted.
❒ ” It also causes the closure of stomata under conditions of water stress as also under high concentration of CO2 in the guard cells. Thus it act as antitranspirant and is also known as Stress hormone.
❒ ” It accelerates senescence of leaves but this effect is reversed by the application of cytokinins in Lemna.
❒” The ABA is a growth inhibitor. It regulates the dormancy of seeds and buds perhaps by inhibiting growth process. The ABA level decreases in the whole seed as their dormancy is broken. The bud dormancy is regulated by a balance of ABA and CK.
❒” It inhibits germination of seeds. This effect is reversed by the action of cytokinin in Lectuca sativa.
❒” It inhibits gibberellin induced growth activities. On account of this antagonastic behaviour, it is called also as antigibberellin.
❒ ” Ethylene is the smallest Phytohormone.
❒ ” It was observed in 1864 that the gas illuminating the streets in German cities, due to leakage in pipes caused leaf fall in the road side shade trees.
❒ ” Girarden (1864) reported that illuminating gas coming out of mines causes defoliation.
❒ ” Neijuber (1901) reported that illuminating gas coming out of mines causes horizontal growth of seedlings.
❒” Crocker (1930) reported presence of ethylene from plant organs and named it as gaseous hormone.
❒ ” Ranjan and Jha (1940) reported that black tip of mango (physiological disease) is caused by gas coming out of brick kiln.
❒” Burg (1962) established that ethylene is the only gaseous growth regulator.
❒” Galston and Davis (1970) recognized ethylene as growth regulator.
❒ ” No algal form is known to synthesize ethylene. However, several fungal and a few bacterial forms are known to produce this gas. All seed plants are known to produce ethylene.
❒ ” The most important site of production is shoot apex. The roots also produce this gas but in much less amount. The nodes produce more ethylene than internodes. The leaves and flowers also produce ethylene particularly more when they senesce and abscise. The young fruits produce little ethylene. The ethylene production increases suddenly just before the respiratory climacteric. This indicates ripening of the fruit. When a fruit ripens it’s respiration rate gradually decreases but it is reversed by a sharp increase called climacteric. Gentle rubbing of leaves and stem, wounding, mechanical stress conditions, viruses, insects and water logging increase ethylene production.
❒ ” The ethylene is synthesized from the amino acid, methionine in many plants like apple, cauliflower, tomato etc. However, it is also thought to be synthesized from isoamyl alcohol, methional, ethyl acetate or glutamate. Its synthesis from glucose via methionine or its aldehyde methional occurs in the following manner:
Glucose → Oxaloacetate → Aspartate → Homocysteine → Methionine → Ethylene
Methionine Methional Ethylene
❒ ” Ripening of Fruits : Unripe fruits can be made to ripe before proper time if they are kept in ethylene atmosphere. Uncontrolled application of this gas may spoil the fruits.
❒ ” It causes petal discolouration. Induction of fading in pollinated orchids is a well known effect of ethylene. Highest rate of release of ethylene from fading flowers Vanda has been reported.
❒ ” It induces epinasty. It is active in very low concentration. Even 1 PPM of this gas can induce epinasty in Rosa leaves.
❒ ” In some plants, it stimulates germination of seed.
❒ ” It inhibits root and stem elongation but induces root hair formation.
❒ ” It induces leaf expansion, seismonasty, synthesis of β-1, 3 glucan and flowering in pine apples.
❒ ” It regulates the cell wall growth and induces a cell to grow isodiametrically.
❒ ” It also induces cellulase activity leading to promotion of leaf abscission.
❒ ” It also suppresses the growth of buds in decapitated stem.
❒ ” Linnaeus thought that plants lack any power of movement but his views were criticized by Darwin who observed movement of tendrils. Botanists at Royal Botanic Gardens, Kew, England observed that Abrus precatorius (rati) not only responds to magnetic and electric responses but can also predict cyclones, volcanic eruptions and earthquakes.
❒ ” The plants respond to various stimuli such as light (photic), temperature (thermal), chemical, electrical, gravitational and contact. Stimulus is a change in external or internal environment of an organism that elicits response in the organisms. The reaction of plant to a stimulus is known as response. The power or ability of a plant to respond to a stimulus is called sensitivity or irritability. The area which perceives a stimulus is called perceptive region while the plant part showing the response is known as responsive region. While in certain lower forms like algae, fungi and bacteria the entire organism moves, in rooted plants the movement is limited to certain organs only. The plants perceive certain stimuli, they are transduced and then a specific response is observed. The events which connect the perception of stimulus to the observed response are covered under the term transduction. The influence of any stimulation may be one of the following types:
❒ Functional: When stimulation increases respiration rate,
❒ Nutritive: When it changes the rate of nutrition,
When it changes the formKINDS OF MOVEMENTS
❒ ” Plant movements occur due to four types of changes : Physical, locomotion, growth and turgor.
❒ ” The plant movements are broadly classified into two categories; vital and physical.
❒ ” They are shown by living cells or plants. They have been subdivided as under:
❒ Movements of Locomotion
➢ Autonomous or spontaneous:
❒ I. Ciliary II. Amoeboid III. Cyclosis
➢ Paratonic or autogenic or induced or tactic:
❒ I. Phototactic II. Thermotactic III. Chemotactic IV. Rheotactic V. Galvanotactic
❒ Movements of Growth
❒ I. Nastic II. Nutational
➢ Paratonic or Tropic
❒ I. Phototropism II. Geotropism III. Hydrotropism IV. Chemotropism
❒ V. Thigmotropism VI. Aerotropism VII. Thermotropism VIII. Rheotropism
❒ IX. Galvanotropism X. Traumatotropism.
❒ Movements of Variation
➢ Paratonic or Nastic
❒ I. Nyctinasty II. Thigmonasty III. Hydronasty IV. Chemonasty V. Seismonasty.