Growth can be defined as a vital process which brings about irreversible permanent change in any plant or its part with respect to its size, form, weight and volume.
Regions of growth : In unicellular plants there is overall growth and not confined to any specific region but in multicellular plants growth is restricted to specific regions having meristematic cells. On the basis of their position in the plant body (higher plants) meristems are divided into three main categories.
(1) Apical meristems : These meristems are found at shoot and root apex. As a result of activity of these meristems plant increases in length. In angiosperms and gymnosperms there is a group of meristematic cells but in bryophytes and pteridophytes there is a single tetrahedral cell found at the shoot apex.
(2) Intercalary meristems : These meristems are found above the nodes. As a result of the activity of these meristems increase in length takes place. e.g., Bambusa.
(3) Lateral meristems. : These meristems are made up of cells which divide in radial direction only. They form laterally placed new cells towards the centre and periphery. Cork cambium (phellogen) and vascular cambium are the examples of lateral meristems. Increase in girth of shoots and roots take place because of the activity of this cambium.
Phases of growth
(1) Cell division (Formative phase) : Growth is based on mitotic cell division.
(2) Cell enlargement : Cell division is followed by cell enlargement. The cell increases in size due to vacuolation (by absorption of water). The cell enlargement has been explained in two different ways. According to the first view, the turgor of the cell is responsible for cell enlargement. The other view considers that as a result of growth of the cell wall the volume of the cell increases.
(3) Cell maturation (Differentiation) : Cell differentiation following cell division and cell enlargement leads to the development of specialized mature tissue cell. e.g., xylem tracheids and trachea, sieve tubes and companion cells.
Growth curve : The rate of growth varies in different species and different organs. The young leaf sheath of banana grows for a time at the rate of almost three inches per hour. Growth begins slowly, then enters a period of rapid enlargement, following which it gradually decreases till no further enlargement occurs. The mathematical curve which represents this variation in growth rate is some what flattened S-shaped curve or sigmoid curve. Time in which growth takes place has been called grand period of growth. This term was coined by Sachs. The analysis of growth curve shows that it can be differentiated into three phases :
(1) Lag phase : The rate of growth is very slow in lag phase. More time is needed for little growth in this phase.
(2) Log phase (Exponential phase) : The growth rate becomes maximum and more rapid. Physiological activities of cells are at their maximum. The log phase is also referred to as grand period of growth.
(3) Final steady state (Stationary phase)or Adult phase : When the nutrients become limiting, growth slows down, so physiological activities of cells also slows down. This phase is indicates maturity of growth system. The rate of growth can be measured by an increase in size or area of an organ of plant like leaf, flower, fruit etc. The rate of growth is called efficiency index.
In many plants another phase is also evident in their growth curve. This is called linear phase or phase of maximum growth rate. Sachs called it as grand phase.
Measurement of growth
The following methods are designed to measure growth in length :
(1) Direct method : Measurement is done between two marked points by a scale at regular intervals.
(2) Horizontal microscope (Travelling microscope)
(3) Auxanometer : Several kinds of auxanometers have been devised to measure the growth in length of a plant. Two of them are :
(i) Arch auxanometer
(ii) Pfeffer’s auxanometer(Automatic auxanometer)
(4) Bose’s crescograph : The crescograph invented by Sir Jagdish Chandra Bose is a more delicate instrument and gives magnification upto 10,000 times. The rate of growth of root can be measured by the use of a root auxanometer.
Factors influencing rate of growth : 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. Physiological 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) 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.
(ii) 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.
(iii) Duration of light : There is remarkable effect on duration 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.
(2) 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.
(3) Water : As water is an essential constituent of the living cell, a deficiency of water causes stunted growth.
(4) 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.
(5) 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.
(6) Pollutants : Several pollutants such as automobile exhaust, peroxyacetyl nitrate (PAN), pesticites etc have detrimental effect on plant growth. 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.
(7) 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.
Internal factors
(1) Nutrition : It provides raw material for growth and differentiation as well as source of energy. C/N (Carbohydrate/Nitrogen) 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.)
(2) 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.
Growth hormones and Growth regulators
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. 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’. A group of plant hormones including auxins, gibberellins, cytokinins, ethylene and abscisic acid are presently known to regulate growth.
Auxins : Auxins (Gk. auxein = to grow) are weakly acidic growth hormones having an unsaturated ring structure and capable of promoting cell elongation, especially of shoots (more pronounced in decapitated shoots and shoot segments) at a concentration of less than 100 ppm which is inhibitory to the roots. Among the growth regulators, auxins were the first to be discovered.
Discovery : Julius Von Sachs was the first to indicate the presence of organ forming substances in plants. The existence of first plant growth hormone came from the work of Darwin and Darwin (1881). Darwin described the effects of light and gravity in his book, “Power of movements in plants”. Darwin and his son found that bending movement of coleoptile of Canary grass (Phalasis canariensis) was due to exposure of tip to unilateral light. Boysen-Jensen (1910; 1913) found that the tip produces a chemical which was later named auxin. Paal (1914, 1919) removed coleoptile tip and replaced it asymmetrically to find a curvature. Auxin was first collected by Went (1928) from coleoptile tip of Avena. Went also developed Avena curvature test for bioassay of auxin.
Types of auxins : There are two major categories of auxins natural auxins and synthetic auxins.
(1) 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 occurring auxin was isolated by Kogl and Haagen-Smit (1931) from human urine. It was identified as auxin-a (auxenotriolic 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. Higher concentration of auxins show inhibitory effect on growth.
(2) 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.
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.
(1) 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.
(2) Split pea stem curvature test : This test was also discovered by Went, 1934.
(3) Root growth inhibition test (Cress root inhibition test)
Functions of auxins : Auxins control several kinds of plant growth processes. These are as follows :
(1) 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.
(2) 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).
(3) Control of abscission layer : Auxin inhibits abscission of leaves and fruits. Abscission layer is produced when the auxin content falls below a minimum.
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.
(4) 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.
(5) 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.
(6) Parthenocarpy : 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.
(7) 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.
(8) 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.
(9) 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.
(10) 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.
(11) 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.
(12) Nodule formation : In legumes, IAA is known to stimulate nodule formation.
(13) 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 : 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.
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.
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.
Many of them occur naturally in plants. Gibberella fujikuroi has as many as 15 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.
Bioassay of gibberellin : Gibberellin bioassay is performed through dwarf maize/pea test and cereal endosperm test.
(1) 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.
(2) 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 endosperms kept as control.
Functions of gibberellin
(1) Stem elongation : The gibberellins induce elongation of the internodes. The elongation of stem results due to rapid cell division and cell elongation induced by gibberellins.
(2) 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.
(3) 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.
(4) 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).
(5) 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, a-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.
(6) 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).
(7) 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.
(8) 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.
(9) 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.
(10) Malt yield : There is increased malt production when gibberellins are provided to germinating barley grains (due to greater production of a-amylase).
(11) Delayed ripening : Ripening of citrus fruits can be delayed with the help of gibberellins. It is useful in safe and prolonged storage of fruits.
(12) Seed germination : Gibberellins induce germination of positively photo-blastic seeds of lettuce and tobacco in complete darkness.
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.
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 Pseudomonastumefaciens. 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 produced 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.
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.
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.,
(1) 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.
(2) 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.
(3) 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.
(4) 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.
Functions of cytokinins
(1) 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.
(2) 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.
(3) 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.
(4) 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.
(5) 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.
(6) Accumulation and Translocation of solutes : Cytokinins induce accumulation of salts inside the cells. They also help solute translocation in phloem.
(7) Sex expression : Cytokinins promote formation of female flowers in some plants.
(8) Enzyme activity : Cytokinins stimulate the activity of enzymes especially those concerned with photosynthesis.
(9) Parthenocarpy : Development of parthenocarpic fruits through cytokinin treatment has been reported by Crane (1965).
(10) Pomalin : A combination of cytokinin (6-benzladenine) and gibberellin (GA4, GA7) called pomalin is particularly effective in increasing apple size.
(11) Initiation of interfasicular cambium : Cytokinins induce the formation of interfasicular cambium in plants e.g., Pinus radiata.
(12) Nucleic acid metabolism : Guttman (1957) found a quick increase in the amount of RNA in the nuclei of onion root after kinetin treatment.
(13) Protein synthesis : Osborne (1962) demonstrated the increased rate of protein synthesis on kinetin treatment.
(14) 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.
Gibberellin is also known to play essential role in germination of cereal seeds.
Ethylene : Ethylene is a gaseous hormone which stimulates transverse growth but retards the longitudinal one.
Discovery : The effect of ethylene had been known since long. Kerosene lamps and hay have been used by fruit merchants to hasten colour development (ripening) in fruits. These effects are due to ethylene. Neljubow (1901) observed that ethylene gas alters the tropic responses of roots. Denny (1924) reported that ethylene induces ripening of fruits. Crocker et al. (1935) identified ethylene as gaseous plant hormone.
Ethylene is produced in plants from the amino acid methionine. It is synthesized in almost all plant parts-roots, leaves, flowers, fruits, seeds. It is more synthesized in nodal regions. Maximum synthesis of ethylene occurs during climacteric ripening of fruits. High concentration of auxin induce ethylene formation. When a fruit ripens its respiration rate gradually decreases, but when it is reversed by a sharp increase called climactric. Some of the inhibitory effects earlier attributed to auxin are known to be caused by ethylene.
The commercial product for providing ethylene is ethaphon (2-chloroethyl phosphoric acid). Ethaphon is a liquid from which ethylene gas is released, hence this substance is used for artificial ripening of fruits.
Bioassay of ethylene : It is done on the principle of triple response which includes three characteristic effects of ehtylene on etiolated seedlings of pea-viz.
Swelling of nodes.
Inhibition of elongation of internodes of stem.
Induction of horizontal growth of stem against gravity.
(1) Triple pea test : Pratt and Biale (1944) developed this method for bioassay of ethylene which base on the physiological effect of ethylene to cause
Subapical thickening of stem.
Reduction in the rate of elongation.
Horizontal nutation (transverse geotropism) of stem in etiolated pea seedlings. In presence of ethylene, epicotyls show increase growth in thickness and reduced rate of longitudinal and horizontal growth.
(2) Pea stem swelling test : Cherry (1973) used pea seedlings to measure ethylene concentration by marked increase of stem swelling expressed as a ratio of weight to length. In one ppm of ethylene the ratio is about 4.0.
Functions of ethylene
(1) Fruit growth and Ripening : Ethylene promotes fruit growth and its ripening. The hormone is used in the artificial ripening of climacteric fruits (e.g., Apple, Banana, Mango).
(2) Transverse growth : Ethylene inhibits longitudinal growth but stimulates transverse growth so that stem looks swollen.
(3) Epinasty (leaf bending) : Epinasty represents more growth on upper surface of leaf than on lower surface. Epinasty is said to be controlled by ethylene in many plants.
(4) Abscission : Ethylene stimulates formation of abscission zone in leaves, flowers and fruits.
(5) Apical dominance : Ethylene inhibits the growth of lateral buds and thus cause apical dominance (in pea). It is believed that auxin might be functioning partly through synthesis of ethylene in causing apical dominance.
(6) Root initiation : In low concentration, ethylene stimulates root initiation and growth of lateral roots and root hair.
(7) Flowering : Ethylene stimulates flowering in pineapple and related plants though in other cases, the hormone causes fading of flowers. Fading flowers of Vanda are known to release ethylene. Sleep disease (inrolling of petals in blossomed flowers) in due to ethylene.
(8) Sex expression : Ethylene application increases the number of female flowers and fruits in cucumber plants.
(9) Dormancy : It breaks dormancy of different plant organs but not of lateral buds.
Abscisic acid (ABA) : Abscisic acid is a mildly acidic growth hormone, which functions as a general growth inhibitor by counteracting other hormones (auxin, gibberellins, cytokinins) or reactions mediated by them.
Discovery : The hormone was first isolated by Addicott et al. (1963) from cotton balls. They named it as abscisin II. Simultaneously, Wareing and Cornforth isolated a substance that can induce bud dormancy. They named the substance as dormin. Later, both these substances were found to be the same and were named as abscisic acid. It is produced in many parts of the plants but more abundantly inside the chloroplasts of green cells. The synthesis of abscisic acid is stimulated by drought, water logging and other adverse environmental conditions. Therefore, it is also called stress hormone. The hormone is formed from mevalonic acid or xanthophylls. Chemically it is dextro-rotatory cis sesquiterpene. The hormone is transported to all parts of the plant through diffusion as well as through conductive channels.
In some plant tissues (especially in young shoots) occurs a related compound called xanthoxine.
Whether xanthoxine is an intermediate of the ABA-biosynthesis or whether it is an independent product remains unknown. The structure indicates that both ABA and xanthoxine are terpene derivatives.
Bioassay of abscisic acid
(1) Rice seedling growth inhibition test : Mohanty, Anjaneyulu and Sridhar (1979) used rice growth inhibition method to measure ABA like activity. The length of second leaf sheath after six days of growth is measured.
(2) Inhibition of a-amylase synthesis in barley endosperm test : ABA inhibits the synthesis of a-amylase in the aleurone layers which is triggered by gibberellins. Goldschmidt and Monselise (1968) developed the bioassay method to estimate ABA activity by determining the extent of inhibition of a-amylase synthesis induced by treating barley seed endosperm with GA.
Functions of abscisic acid
(1) Control : It keeps growth under check by counter acting the effect of growth promoting hormones, i.e., auxins, cytokinins and gibberellins. As growth is primarily controlled by gibberellins, abscisic acid is popularly called antigibberellic hormone. It will inhibit seed germination, growth of excised embryos, growth of Duckweed and other plants.
(2) Dormancy : Abscisic acid acts as growth inhibitor and induces dormancy of buds towards the approach of winter. Dormancy of seeds is mainly caused by abscisic acid. Because of its action in inducing dormancy abscisic acid (ABA) is also called dormin. The buds as well as seeds sprout only when abscisic acid is overcome by gibberellins.
(3) Abscission : ABA promotes the abscission of leaves, flowers and fruits in plants.
(4) Senescence : Abscisic acid stimulates senescence of leaves by causing destruction of chlorophyll (an effect opposite to that of cytokinins) and inhibition of protein and RNA synthesis. The effect, however, can be reversed by application of cytokinins in Lemna.
(5) Antitranspirant : Abscisic acid can be used as antitranspirant. Application of minute quantity of ABA to leaves reduces transpiration to a great extent through partial closure of stomata. It thus conserves water and reduces the requirement of irrigation.
(6) Hardiness : Abscisic acid promotes cold hardiness and inhibits growth of pathogens.
(7) Flowering : ABA delays flowering in long day plants. However, in some short day plants (e.g., strawberry, black current) it promotes flowering.
(8) Rooting : Abscisic acid can be used to promote rooting in many stem cuttings.
Wound hormone or Traumatic acid or Necrohormone : Haberlandt (1913) reported that injured plants cells release a chemical substance (wound hormone), which stimulate the adjacent cells to divide rapidly in order to heal up the wound. English et al. (1939) finally isolated and crystallized this wound hormone and named it as Traumatic acid. Although traumatic acid has been found to be very active in inducing meristematic activity in uninjured green bean pods, but it is not effective in most of the plant tissues including tobacco pith tissues.
Morphactins : Morphactins are synthetic growth regulators which act in variety of ways on the natural regulation mechanisms of plants. The important ones are phenoxyalkancarboxylic acid (synthetic auxin), substituted benzoic acids, Malic acid hydrazide, Fluorene-9 carboxylic acids and their derivatives, Chlorflurenol, Chloroflurun, Flurenol, Methylbenzilate, Dichlorflurenol, etc. Morphactins have fundamental action on morphogenesis of plants and this characteristic designation (morphactins) is derived from morphologically active substances.
The actions of these substances are systematic and after their uptake they are transported and distributed not polarly (as seen by IAA) but basi- and acropetally. Generally these are growth inhibitors. These contain ‘fluorene ring’ in their structure.
Functions
(1) Seed germination : In general, morphactins inhibit germination seeds particularly the emergence of the radicle from the seed shell. This property can be counteracted with GA3 and almost completely by cytokinins. The germination of fern spores is also delayed by morephactins.
(2) Growth seedling : Morphactins inhibit the growth of seedling affecting the shoot and often also root. With this property they show a similarity with cytokinin. The inhibitory effect of seedling shoot growth can be partly counteracted with GA3 but not the inhibition of root growth.
(3) Stem elongation : They have inhibitory effect on the stem elongation. Increased concentration produces dwarfing in the plants.
(4) Polarity of cell division : Denffer and others (1969) observed in the dividing cells of the root tips of Allium that treatment of morphactin (CFI) results in random orientation of the mitotic spindle and plane of cell division, i.e., they exercise depolarisation during cell division.
Jasmonic acid (Jasmonates) : According to Parthier (1991), jasmonic acid and its methyl esters are ubiquitous in plants. They have hormone properties, help regulating plant growth, development and they seem to participate in leaf senescence and in the defense mechanism against fungi.
Just like ABA jasmonates inhibit a premature germination of the oil-containing seeds of Brassica and Linum. After germination they induce the synthesis of the seed storage proteins Napin and Cruciferin as well as that of several more elaiosome-associated proteins.
Calines (Formative hormones) : Certain other natural growth hormones in plants called as calines or formative hormone which are through to be essential for the effect of auxin an root, stem and leaf growth they are :
(1) Rhizocaline or Root forming hormone : It is produced by the leaves and translocated in a polar manner down the stem.
(2) Caulocaline or Stem forming hormone : It is produced by the roots and is transported upward in the stem.
(3) Phyllocaline or Self forming hormone : It is produced probably by the cotyledons. It stimulates mesophyll development in the leaves and is synthesized only in the presence of light.
Physiology of flowering plant
Flowering in a plant occurs at a particular time of the year and controlled by many morphological and environmental conditions. Two important controlling factors are photoperiod or light period, i.e., photoperiodism, low temperature i.e., vernalization.
(1) Photoperiodism (Light period) : The effects of photoperiods or daily duration of light periods (and dark periods) on the growth and development of plants, especially flowering is called photoperiodism. The role of photoperiodism in the control of flowering was demonstrated for the first time by W.W Garner and H.A. Allard (1920). They observed that Maryland Mammoth variety of tobacco could be made to flower in summer by reducing the light hours with artificial darkening. It could be made to remain vegetative in winter by providing extra light. On the basis of length of photoperiod requirements of plants, the plants have been classified into following categories.
(i) Short day plants (SDP) : These plants initiate flowering when the day length (Photoperiod) become shorter than a certain critical period. Most of winter flowering plants belong to this category e.g., cocklebur (Xanthium), Chrysanthemum, sugarcane, tobacco (Mutant Maryland Mammoth), soyabean, strawberry etc.,
(ii) Long day plants (LDP) : These plants begin flowering when the day length exceeds a critical length. This length too differs from species to species. The long day plants fail to flower, if the day length is shorter than the critical period. e.g., spinach (Spinacea oleracea), henbane (Hyoscymus niger), radish, sugar-beet, wheat, lattuce, poppy, larkspur, maize etc.
(iii) Day neutral plants : These plants can flower in all possible photoperiods. The day neutral plants can blossom throughout the year. e.g., cucumber, cotton, sunflower, tomato, some varieties of pea, etc.
(iv) Intermediate plants : These plants flower only under day lengths within a certain range usually between 12-16 hours of light but fail to flower under either longer or shorter photoperiods. e.g., Mikania scandens, Eupatorium hyssopifolium and Phaseolous polystacous.
(a) Amphiphotoperiodic plants : Such plants remain vegetative on intermediate day length and flower only on shorter or longer day lengths. e.g.,Mediaelegans.
(b) Short long day plants : These plants require short photoperiods for initiation of flowering and long photoperiods for blossoming. e.g.,Triticum vulgare, Secale cereale.
(c) Long short day plants : These plants require long photoperiods for initiation of flowering and short photoperiods for blossoming. e.g.,Bryophyllum, Cestrum.
Critical period : Critical photoperiod is that continuous duration of light, which must not be exceeded in short day plants and should always be exceeded in long day plant in order to bring them to flower. There is no relation with the total day length. Thus, the real distinction between a SDP and LDP is whether flowering is induced by photoperiods shorter or longer than the critical period. The critical day length for Xanthium (a short day plant) is 15. 6 hours and that for Hyoscymus niger (a long day plant) is about 11 hours, yet the former is SDP as it flowers in photoperiods shorter than its critical value, whereas the latter is LDP requiring photoperiods longer than its critical value. Both Xanthium and Hyoscymus niger flower with 14 hours of light per day. Thus, day length in which a plant flowers is no indication of its response class in the absence of further information.
Skotoperiodism (Dark period) : When photoperiodism was discovered, the duration of the light period was thought to be critical for flowering. Subsequently, it was found that when the long night period was interrupted by a brief exposure to light, the short day plants, failed to flower. Thus, for flowering, these plants require a long night or critical dark period rather than a short day length. Similarly, long day plants respond to nights shorter than the critical dark period. Curiously, they do not need an uninterrupted dark period. Therefore, a short day plant is also called long night plant and a long day plant as a short night plant.
In the night interruption experiments, when the short day plants were exposed to a flash of light before achieving a critical dark period, flowering was prevented. It is called light break reaction.
Mechanism of photoperiodism
Photoreceptor : The chemical which perceives the photoperiodic stimulus in leaves is phytochrome. The wavelengths of light are absorbed by the leaves. This becomes evident by the fact that defoliated (leaves removed) plant does not flower. Presence of even a single leaf is sufficient to receive required amount of photoperiod. Partially mature leaves are more senstitive to light while very young or mature leaves are much less sensitive to photoperiodic induction.
Garner and Allard’s early work led to the discovery, isolation and much of the characterization of the pigment responsible for absorbing light involved in photoperiodic phenomenon of plants. Borthwick, Hendricks and their colleagues later termed this pigment phytochrome. Pigment was isolated by Butter et al. (1959). This pigment controls several light dependent developmental processes in plants besides flowering, phytochrome exist in two interconvertible forms. The red (660nm), absorbing form Pr and the far red (730 nm), absorbing form Pfr. Pr is converted to Pfr on absorbing red light. Pfr is converted to Pr rapidly absorbing far red light or slowly in darkness. The slow conversion to red absorbing form is under thermal control. During the day when white light available, Pfr accumulates in the plant. This form of phytochrome is inhibitory to flowering in short day plants and stimulatory to flowering in long day plants. In evening, Pfr undergoes thermal and spontaneous decay to change into Pr. This pigment is stimulatory to flowering in short day plants and inhibitory to flowering in long day plants.
Therefore, in SDP interruption of dark periods with a flash of red light converts Pr into Pfr and flowering is inhibited.
Importance of phytochrome : Phytochrome is located in plasmamembrane. Phytochrome far red (Pfr) form is considered to be biologically active form and is responsible to initiate a number of physiological process such as.
(1) Elongation of stem and leaves.
(2) Plastids morphology and differentiation of stomata.
(3) Seed germination.
(4) Photoperiodism and transpiration.
The florigen complex (Flowering hormone): When the proper amount of light is perceived by leaves, they produce a chemical (flowering hormone), which undergoes stabilisation in dark. Later on, this chemical passes to shoot apex and causes its differentiation into flowering shoot.
Chailakhyan (1936) a Russian investigator on photoperiodism, proposed that it be called ‘florigen’. According to him (1958) the “Florigen complex”, the true flowering hormone includes two groups of substances formed in leaves :
Gibberellins : Which are necessary for formation and growth of stem.
Anthesins : Substances which are necessary for flower formation.
Photomorphogenesis : When plants are grown in continuous darkness they become etiolated i.e., such plants are longer, weaker, having yellowish half opened leaves, while light grown plants do not show such conditions. When etiolated plants are kept in light they gradually develop green colour and become normal. The effect of light in reversing etiolation involves two kinds of action; one the biochemical level for the synthesis of the chlorophyll and secondly at the level of morphogenesis light acts to promote expansion of the leaves and inhibits elongation of the internodes. This phenomenon is called photomorphogenesis and is independent of the direction of light.
The action spectrum of photomorphogenesis reveals that plants are most sensitive to red light, but blue light is ineffective.
(2) Vernalization : Russian agronomist Lysenko coined the term vernalization (1929-30). According to him vernalization may be defined as the method of inducing early flowering in plants by pretreatment of their seeds at low temperatures. Chourad (1960) has defined it as the acquisition or acceleration of the ability to flower by chilling treatment. The low temperature requirement for flowering was first noticed by Klipport (1857) while working with winter varieties of cereals such as wheat, barley, oat and rye. He observed that, these varieties when sown in spring failed to flower the same year but grow vegetatively. Such winter varieties, when sown in the autumn, they flowered in spring of the same year.
Site of vernalization : The stimulus of vernalization is perceived only by the meristematic cells such as shoot tip, embryo tips, root apex, developing leaves etc.
Requirement of vernalization :
(i) Low temperature: Low temperature required for vernalization is usually 0-4oC is most of the cases. The chilling treatment should not be immediately followed by high temperature (i.e., about 40oC), otherwise the effect of vernalization is lost. This phenomenon is called de-vernalization.
(ii) Duration of low temperature treatment : It varies from species to species from a few hours to a few days.
(iii) Actively dividing cells : Vernalization stimulus is perceived only by actively dividing cells. Therefore, vernalization treatment can be given to the germinating seeds or whole plant with meristematic tissues and other conditions.
(iv) Water : Proper hydration is must for perceiving the stimulus of vernalization.
(v) Oxygen : Aerobic respiration is also a requirements for vernalization. The stimulus has been named as vernalin (reported by Mechlers).
Process of vernalization : Usually vernalization treatment is given to the germinating seeds. The seeds are moistened sufficiently to allow their germination. They are then exposed to a temperature of 0-4oC for a few weeks and sown to the fields. Lysenko developed the process of vernalization it is completed in two stages.
(i) Thermostage : Germinating seeds are treated with 0-5oC in presence of oxygen and slight moisture. The seed dormancy is broken.
(ii) Photostage : The stage is very essential to initiate the reproductive phase. After vernalization plants must be subjected to a correct photoperiod in order that they may produce flowers.
Importance of vernalization
(i) Vernalization is believed to overcome some inhibitor and induce synthesis of growth hormones like gibberellins.
(ii) It reduces the vegetative period of plant.
(iii) It prepares the plant for flowering.
(iv) It increases yield, resistance to cold and diseases.
(v) Vernalization can remove kernel wrinkles in wheat.
(vi) Vernalization is beneficial in reducing the period between germination and flowering. Thus more than one crop can be obtained during a year.
Senescence and Death
Plant and their parts develop continuously from germination until death. The production of flowers, fruits and seeds in annuals and biennials leads to senescence. The latter part of the developmental process, which leads from maturity to the ultimate complete loss of organization and function is termed senescence. Several workers equate ageing and senescence as same process. Ageing is a sum total of changes in the total plant or its constituents while senescence represents degenerative and irreversible changes in a plant. The study of plant senescence is called phytogerontology.
Types of senescence : Plant senescence is of four types- whole plant senescence, shoot senescence, sequential senescence and simultaneous senescence. The last three are also called organ senescence.
(1) Whole plant senescence : It is found in monocarpic plants which flower and fruit only once in their life cycle. The plants may be annual (e.g., rice, wheat, gram, mustard etc.), biennials (e.g., cabbage, henbane) or perennials (e.g., certain bamboos). The plant dies soon after ripening of seeds.
(2) Shoot senescence : This type of senescence is found in certain perennial plants which possess underground perennating structures like rhizomes, bulbs, corm etc. The above ground part of the shoot dies each year after flowering and fruiting, but the underground part (stem and root) survives and puts out new shoots again next year. e.g., banana, gladiolus, ginger etc.
(3) Sequential senescence : This is found in many perennial plants in which the tips of main shoot and branches remain in a meristematic state and continue to produce new buds and leaves. The older leaves and lateral organs like branches show senescence and die. Sequential senescence is apparent in evergreen plants e.g.,Eucalyptus, Pinus, Mango.
(4) Simultaneous or synchronous senescence : It is found is temperate deciduous trees such as elm and maple. These plants shed all their leaves in autumn and develop new leaves in spring. Because of this shedding of leaves, autumn season is also called fall. e.g., Dalbergia, Elm, Mulberry, Poplar.
Theories of senescence
(1) Wear and tear : According to this theory, senescence occurs due to loss of activity and cells undergo wear and tear due to disintegration of organelles.
(2) Toxicity : It is viewed that senescence takes place due to accumulation of toxic and deleterious substances in all.
(3) Loss of metabolites : It is assumed that senescence leads to gradual depletion of essential metabolites in a cell.
(4) Genetic damage
Characteristics of ageing and senescence
(1) There is general decline in metabolic activities, decline in ATP synthesis and also decreased potency of chloroplast.
(2) Decrease in RNA and DNA
(3) Decrease in semipermeability of cytoplasmic membranes.
(4) Decrease in the capacity to repair and replace wornout cells.
(5) There may be accumulation of chromosomal aberrations and gene mutations with advancing age as a result of these changes protein synthesis becomes defective.
(6) Increased production of hydrolytic enzymes such as proteases and nucleases.
(7) Deteriorative change in cell organelles and membranes.
(8) Decrease in the internal content of auxin and cytokinins and increase in the production of abscisic acid or ethylene.
Importance of senescence : Biologically senescence and death have following advantages :
(1) It maintains efficiency since the old and inefficient organs are replaced by young efficient part like leaves, buds, flowers and fruits. etc.
(2) During senescence, the cellular breakdown results in release of many nutrients including amino acids, amides, nucleotides, simple sugars and minerals. The same are withdrawn from the senescing organs into the main trunk and later utilised in the growth and developed of new parts.
(3) Shoot senescence is a mechanism to help the plants perennate during the unfavourable periods.
(4) Simultaneous or synchronous leaf fall occurs in autumn prior to winter. It reduces transpiration, which is essential for survival in winter, when the soil in frozen and roots can not absorb water.
(5) Litter of fallen leaves and twigs is an important source of humus and mineral replenishment for the soil.
Abscission
The process of shedding of leaves, fruits or flowers by a plant is called abscission. The shedding of plant parts takes place by the formation of a special layer of cells called abscission layer, within the region of attachment. The middle lamella between certain cells in this layer in often digested by polysaccharide hydrolyzing enzymes such as cellulase and pectinases.
Plant movements
Plants show movements in response to a variety of stimuli. Stimulus can be defined “as a change in external or internal environment of an organism that elicits response in the organism”. 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 reactivity or irritability.
The movements which occur without the effect of external stimulus are called autonomic or spontaneous movements. Thus spontaneous movements are brought by definite internal stimulus, and if the movements are produced in response to external stimulus, they are known as paratonic or induced movements.
The area which perceives a stimulus is called perceptive region, while the plants part showing the response is known as responsive region. The minimum duration or time required for a stimulus to be applied continuously on the perceptive region to produce visible response is called presentation time. The duration between the application of stimulus and production of visible response is called latent time or reaction time.
Classification of plant movements
Plants movements are broadly classified into two types:
(1) Movements of locomotion: In this case, plant moves physically from one place to another. The movements of locomotion are of two type-autonomic (occurs spontaneously) or paratonic (induced by external stimuli).
(i) Autonomic movement of locomotion: These movement of locomotion are due to internal stimuli they are of following types :
(a) Ciliary movements : Certain motile algae (e.g.,Chlamydomonas, Volvox, etc). Zoospores and gametes of lower plants move from one place to another by means of cilia or flagella.
(b) Amoeboid movements : It is the movement of naked mass of protoplasm by means of producing pseudopodia like process e.g., members of Myxomycetes (slime fungi).
(c) Cyclosis : These are movements of cytoplasm with in a cell (also called protoplasmic streaming). These are of two types :
Rotation : When the protoplasm moves around a single central vacuoles in either clockwise or anticlockwise direction g., leaf cells of Hydrilla, Vallisneria.
Circulation : When the movement of protoplasm accurs around different vacuoles in different directions within the cell g., staminal hair of Tradescantia, shoot hairs of gourds.
(d) Excretory movements : Apical part of Oscillatoria is like a pendulum. It is considered that such movements are due to excretion of substances by the plants. (movements opposite to the side of excretion).
(ii) Paratonic movement of locomotion (Tactic movement) : These movements take place in whole small plants. e.g., chlamydomonas or small free ciliated organs e.g., gametes. These movements are due to external factors like light, temperature or chemicals and are of following types :
(a) Phototactic movements or phototaxisms : It is the movement of free living organism towards or away from light. e.g., movement of Chlamydomonas, Ulothrix, Cladophora, Volvox etc. towards suitable light intensity. Three types of arrangement present in columular cells in chloroplast of dorsiventral leaves.
Parastrophe : In intense (maximum) light chloroplasts of cells arranged in longitudinal wall as a sequence manner.
Apostrophe : In minimum light chloroplasts of cells arranged in different manner.
Epistrophe : In dark chloroplasts of cells are arranged in transverse wall as sequence manner.
(b) Chemotactic movements or chemotaxisms : It is the movement of plant or plant parts from one place to another towards or away from chemical substance. e.g., male gametes (antherozoids) of bryophyta move towards archegonia under the influence of sugars produced by neck canal cells and also in pteridophyta male gametes move towards archegonia due to the malic acid produced by disintegration of neck canal cells and ventral canal cells.
(c) Thermotactic movements or thermotaxism : It is the movement of free living organism in response to external stimuli of temperature. e.g., Chlamydomonas move from cold water to medium warm water and from very hot water to medium temperature.
(2) Movement of curvature : In these cases, plants are fixed, thus they fail to move from one place to another. Somehow, movement is noticed in the form of bend or curvature on any part of the plant. Movement of curvature can be classified into.
(i) Mechanical or Hygroscopic movements : These movements depend upon the presence or absence of water and occurs in non-living parts of plants. It is of two types.
(a) Hydrochasy : This movement occurs due to the absorption of water.
Example : Peristomial teeth of moss protrude out when the capsule is dry and curve when capsule is wet.
Spores of the Equisetum coil and uncoil in the presence and absence of water respectively.
(b) Xerochasy : This movement occurs due to the loss of water.
Example : When water is lost from the annulus of the sporangia of fern, it burst from stomium and spores are thus liberated out.
(ii) Vital movement : These movement are of two types :
(a) Growth movements : These movements are due to unequal growth in different parts of an organ and are irreversible. They are further divided into two types :
Autonomic growth movements
(i) Nutation (Nutatory movements) : These movements occur in the growing stem of twiners and tendrils. The stem exhibits a kind of nodding movements in two directions. This is because the stem apex shows more growth on one side at one time and a little later there is a greater growth on the opposite side. It is called nutation. In spirally growing stems the region of greater growth passes gradually around the growing point resulting in the spiral coiling of stem and tendrils. Such a movement is called circumnutation. Coiling of a tendril after coming in contact with a support is a thigmotropic movement.
(ii) Nasty movements : They are non-directional movements in which the response is determined by the structure of the responsive organ and not the direction of the stimulus. The responsive organ has an asymmetrical or dorsiventral structure. Greater growth on one side causes the organ to bend to the opposite side. Greater growth on the adaxial side is called hyponasty. e.g., circinate coiling and closed sepals and petals in a floral bud. Whereas more growth on abaxial side is called epinasty. e.g., opening of fern leaf and spreading of sepals and petals during opening of the floral bud.
Paratonic growth movement (Tropic and nastic movements) : These are movements of curvature brought about by more growth on one side and less growth on the opposite side of plant organ induced by some external stimuli. Depending upon the nature of stimuli these movements are of the following type :
(i) Phototropism (Heliotropism) : When a plant organ curves due to unilateral light stimulus it is called phototropism. Some parts of the plant e.g., stem moves towards light. These organs are called positively phototropic. Some other organs e.g., roots move away from light and they are called negatively phototropic. If we keep a plant in a dark chamber (Heliotropic chamber) with an opening on one lateral side the stem tip moves towards light i.e., towards opening. Phototropism of stem and root are due to differential hormonal effect. Violet blue light is most effective. Photoreceptor seems to be a carotenoid. Young stems are positively phototropic, leaves diaphototropic, shoots of Ivy plagio-phototropic, roots either non phototropic or negatively phototropic (e.g., white mustard, Sunflower). Mechanism is believed to be Cholodny-Went theory which states that unilateral light produces more auxin (IAA) and hence more growth on the shaded side resulting in bending.
(ii) Geotropism (Gravitropism) : Growth of movements induced by the stimulus of gravity are known as geotropism.
Generally, the primary root grows towards the force of gravity and hence is positively geotropic. The stem coloptile and pneumatophores grows away from the force of gravity and is negatively geotropic. The secondary roots and stem branches arise at angle less than 90o. They are thus plageotropic. Certain undergorund stems such as rhizomes, stolons of potato are oriented at right angle to the direction of force a gravity and are called diageotropic. Some of the lateral organs (e.g., corolloid roots of Cycas) possess little or no geotropic sensitivity, they are called ageotropic.
If some seedlings are kept in a dark chamber in different directions, root always move downwards and shoot away from the gravitational force.
According to Cholodny-Went theory there is more auxin on the lower side of both stems and roots. In stem higher auxin concentration increases growth while in roots it inhibits growth. Therefore, stem grow more on the lower side while roots grow more on the upper side causing the stem to bend upwardly and roots to bend downwardly. Another theory is statolith theory which states that perceptive regions contain statoliths (microscopic particles). Change in their position causes irritation and hence differential growth. Clinostat / Klinostal is a instrument which can eliminate the effect of gravity and allow a plant to grow horizontly by slowly rotating it.
The main axis of which is attached to a rod. On the top of the rod is attached a flower pot. The clinostat is kept in a horizontal position. When the clock axis rotates the flower pot also rotates. As a result of this the plant grows horizontally as the effect of gravity is nullified by clinostat. If the clock of the clinostat is stopped the rotation of the plant stops, the shoot apex moves upward (negative geotropism) and the root apex moves downwards (positive geotropism).
(iii) Hydrotropism : Growth movements in response to external stimulus of water are termed as hydrotropism. Roots are positively hydrotropic (i.e., bend towards the source of water).
Stem are either indifferent or negatively hydrotropic. Positive hydrotropic movement of the roots is stronger than their geotropic response. In case of shortage of water, roots bend towards the sewage pipes and other sources of water in disregard to the stimulus of gravity.
(iv) Thigmotropism (Haptatropism) : The movement which are due to contact with a foreign body. It is most conspicuous in tendrils which coil around support and help the plant in climbing. e.g., Tendrils of cucurbitaceae, petiole of clematis, leaf apex of Gloriosa.
(v) Chemotropism : When a curvature takes place in response to a chemical stimulus. The growth of pollen tube through stigma and style towards the embryo sac occurs with the stimulus of chemical substances present in the carpel or movement of fungal hyphae towards sugars and peptones.
(vi) Thermotropism : Curvature of plant parts towards normal temperatures from very high or very low temperatures. e.g., peduncles of Tulip, Anemone.
Variation movements (Turgor movements) : These movements are caused by turgor changes especially due to efflux and influx of K+ ions. (swelling or shrinkage of living cells due to change in osmotic potential) and are reversible. Variation movements are further divided into two types :
(1) Autonomic variation movement : These movement of variation, which occurs without the external stimulus. Rhythmic autonomic turgor changes produce jerky rising and falling of two lateral leaflets in Indian Telegraph plant (Desmodium gyrans). Here, large thin walled motor cells found at the leaflet bases regularly lose and gain water bringing about changes in turgor pressure.
Motor (Bulliform) cells present in the epidermal cells of some grasses cause their folding and unfolding movements (hydronasty).
(2) Paratonic variation movement (Nastic movements): These movements of variation are determined by some external stimuli such as light, temperature or contact but the direction of response is prefixed (not determined by the direction of stimuli). Nastic movements are of the following types :
(i) Nyctinastic (sleeping) movements : The diurnal (changes in day and night) movements of leaves and flowers of some species which take up sleeping position at night are called nyctinastic movement. Depending upon the stimulus they may be photonastic (light stimulus) or thermonastic (temperature stimulus). Maranta (Prayer plant), an ornamental house plant provides most common examples of nyctinastic response.
(ii) Photonastic movements : Leaves of Oxalis take up horizontal position in sunlight and droop down during night. Many flowers open during the day and close during night or cloudy sky e.g., Oxalis.
(iii) Thermonastic movements : Flowers of tulips and crocus open during high temperatures and close down during low temperatures.
(iv) Thigmonastic (Haptonastic) movements : When marginal glandular hair of Drosera come in contact with some foreign body e.g., body of insect, they show haptonastic movements. Due to this the insect comes in contact with the central glandular hair which after being stimulated bring the marginal glandular hair on the body of insect. These later movements are chemotropic whereas the previous movements of marginal glandular hair is chemonastic movement Drosera shows both nyctinasty and thigmonasty movements.
(v) Seismonastic movements : This type of movement is brought about in response to external stimulus of shock or touch. The best example of seismonastic movement is the leaves of sensitive plant Mimosa pudica (Touch me not). It shows both nyctinastic (Sleeping movement) and seismonastic movement (shock movements).
? The double sigmoid growth curve occurs in some fruits e.g., Grapes, plum.
? Measurement of growth in young root by making it at 1mm intervals with Indian Ink was first done by Strasburger
? The development of shoot and root is determined by cytokinin and auxin ratio.
? Mixture of 2, 4-D and 2, 4, 5-T (dioxin) is given the name ‘Agent orange’ which was used by USA in Vietnam war for defoliation of forests (i.e., in chemical warfare).
? In glass houses when plants are kept on artificial light and temperature, then this method is called phytotron and is applicable in agriculture, horticulture and tissue culture.
? When each meristem influences other meristems then this phenomenon is called growth correction.
? ABA is used in dryfarming.
? Malic hydrazide is a growth retardant which checks cell division. So during seed storage this is applied for checking sprouting of potato tubers so that the importance of potato may be lowered down.
? Auxin and Cytokinin in combined form shows synergistic effect (affects development of physical structure).
? SDP’s contain anthesins and synthesize gibberellic acid for flowering. Whereas LDP’s contain GA and synthesize anthesins for flowering.
? Leaves show maximum expansion in violet light.
? Knott (1934) found that the locus of photoperiodic induction is the leaves.
? Wellensick (1964) found that the locus for perception of cold treatment is the meristmatic cells (at all places) especially the shoot apex .
? Reduced availability of auxin stimulates leaf fall while presence of auxin slows down leaf fall. Cytokinin prevent senescence through stimulating anabolic activity. They are called antiageing hormones Florigen hormone synthesized in the leaves.
? Physical movements : Unstimulated movements caused by mechanical tensions (e.g., dehiscence of Balsam, clam and Squirting cucumber fruits) and hygroscopicity (Shrinkage/ xerochasy and swelling/ hydrochasy, e.g., dehiscence of fern sporangium,peristome teeth of moss).
? Geotropic stimulus is perceived by root cap in case of root by stem apex in case of stem.
Growth
Maximum growth in roots occurs
(a) At apex (b) In presence of light
(c) Behind the apex (d) In presence of soil
Auxanometer is meant for
[BHU 1987; CPMT 1975, 81, 95, 2002;
Kerala CET 2003; AMU 2005]
(a) Aseptate and multinucleate (b) Septate and multinucleate (c) Aseptate and uninucleate (d) Septate and uninucleate Answer. (b) The mycelium of Aspergillus consist of branch septate hyphae. Cells of hyphae are multinucleate. (184)
(a) Parenchyma (b) Chlorenchyma (c) Plectenchyma (d) Collenchyma Answer. (c) Plectenchyma (Pseudoparenchyma) is nothing but the interwoven mass of fungal mycelium which in cross-section look like parenchyma. (104)
(a) Colletotrichum falcatum (b) Phytophthora infestans (c) Potato mosiac virus (d) Alternaria solani Answer. (b) Late blight of potato is a seed born disease which is caused by Phytophthora infestans. The disease is characterised by brownish to blackish dead areas …
