Growth : Growth may be defined as a vital irreversible process which brings about a permanent change in any plant in respect of its size, form, weight and volume.
Cellular growth has three main aspects as phases of growth. These are—
(i) Cell division : It is the first step of growth of a plant and carries out its growth by cell division of the preexisting cells. Cell division takes place through mitosis which results into the formation of many genetically similar cells.
(ii) Cell enlargement : This phase decides the size of the tissue and organ. In the beginning, cell enlargement is in all directions but in later stages enlargement remains confined to a particular direction.
(iii) Cell differentiation : In this phase of growth newly formed cells undergo structural and physiological changes, depending upon the function they have to perform.
The total period of growth is called grand period of growth. In graph it is called grand period curve, which is always sigmoid in nature so also known as sigmoid growth curve. This curve has three main parts—
(i) Lag phase : Preparatory phase.
(ii) Log phase : exponential growth phase.
(iii) Senescence phase : stationary growth phase.
Measurement of growth : The simplest method of the measurement of growth is by a scale at regular intervals from the beginning to the end. In other methods, horizontal microscope, auxanometers, Bose’s crescograph, and space marker disc are used for the measurement of growth.
Factors affecting growth
Environmental and physiological aspects affect the growth such as absorption of water and minerals, photosynthesis, respiration etc., and environmental factors including climatic and edaphic factors both.
(i) Food supply : The rate of growth is directly proportional to the food supply and with deficient food supply the rate of growth decreases and ultimately stops.
(ii) Water supply : Water supply has a direct relationship with the rate of growth because it is necessary for all the metabolic activities of protoplasm.
(iii) Oxygen supply : Supply of oxygen increases growth because it helps in respiration to convert potential energy into kinetic energy needed for vital activities of the plant including growth.
(iv) Temperature : Temperature affects growth directly or indirectly. The optimum activity takes place at the temperature from 28 to 33OC.
(v) Light : Light intensity, quality and periodicity affects the growth.
(a) Intensity of light : In general high light intensities induce dwarfening of the plants. Plants at the hill tops are short whereas those of a valley are quite tall. Very weak light reduces the rate of overall growth and photosynthesis.
(b) Quality of light : The different wavelengths affect the growth of plant. In blue-violet colour light, internodal growth is pronounced, while green colour reduces the expansion of leaves as compared to complete spectrum of visible light.
(c) Duration of light : There is remarkable effect of the duration of light.
Plant growth hormones : Hormones control growth as well as other physiological processes. These are required in minute quantities and are called plant hormones or phytohormones or growth promoting substances.
All phytohormones are growth regulators but all growth regulators are not phytohormones. Growth regulators may be either growth promoters or growth inhibitors. For balanced and controlled development of plant, growth accelerators (promoters) and growth inhibitors both are essential.
Growth promoters are—
1. Auxins
2. Gibberelins
3. Cytokinins
Growth inhibitors are—1. Abscisic Acid, 2. Ethylene.
Auxins : Auxins are acids. These are three— a—auxentriolic acid—(C18H32O5) b—auxenolonic acid (C18H30O4) IAA—Indole-3-acetic acid (C10H9O2N)
Auxintranslocation : In 1928, Went reported that auxin moves from apical to basal end. Speed is 1 to 1.5 cm/hr and in roots it is 0.1 to 0.2 cm/hr. Applications of auxins
(i) Germination : In India, Chakravarty (1958) used auxins to break the dormancy of seed.
(ii) Rooting : It was suggested in 1934 by Went that root forming substance is identical to IAA.
(iii) Flowering : Auxins play florigenic role in day neutral plants.
(iv) Parthenocarpy : In 1962 Das worked to produce seedless fruits in several plants such as water melon, cucumber etc.
(v) Fruit Setting : In India, fruit setting study of plants has been done on cucurbitaceae, solanaceae, etc.
(vi) Prevention of premature drop of fruits : A successful study on apple, pear, citrus, etc. has been done by using 2, 4–D, 1AA.
(vii) Tissue and Organ Culture : Maheshwari in 1957 produced culture of ovule of Papave somniferum using 1AA and kinetin.
(viii) Weed control : Auxins are weed killers and are considerably used in the destruction of weeds in crop fields, rail-road sides, lawns, and forests.
(ix) Auxins as inhibitors : Large concentrations of auxins inhibit the growth and produce toxic effect on plants.
Gibberellins : The role of auxins as growth hormone of gibberellins came into prominance only after 1950. Yabuta and Hayashi (1939) gave name gibberellin to an active heat labile substance extracted from seedlings suffering from backanae disease. They were able to isolate an active substance from fungus (Gibberella fujikuroi) and called it Gibberellin A.
Later on the discovery was confirmed by Mitchell, Stodola, Brian, et al. Its structure was proposed by Cross, et al. (1961).
Chemical nature of gibberellins : Gibberellins are cyclic diterpenes. These are represented by molecular formula such as C19H24O6 (GA1), C19H26O6 (GA2), C19H22O6 (GA3 or gibberellic acid), C19H24O5 (GA4), etc.
Today about 60 different types of gibberellins are known and they are named as GA1, GA2, GA3 upto GA60.
Mechanism of gibberellin action: The properties of these to influence stem growth and fruit suggested that it might be a form of auxin. A possible mode of action is that GA acts at the gene level to cause derepression of specific genes.
Role of gibberellins
1. Apical bud dormancy : Under certain environmental conditions when the apical buds become dormant and meiotic activity ceases, gibberellin can reverse this dormancy.
2. Role in sub-apical meristem : The role in sub-apical meristem is direct. It regulates the mitotic processes in this region.
3. Cell elongation : Recent findings suggest that auxin exerts the principal control over cell elongation in the stem and gibberellin has a relatively minor role to play.
4. Fruit growth : Crane is 1964 using gibberellin, has produced normal looking seedless fruits from the unfertilised flowers of tomato and from certain varieties of apple and peaches.
5. Flowering : Gibberellins play an important role in the initiation of flowering.
6. Mobilisation of foods in storage cells of seed : Akazawa and Miyata (1982) suggested that GA stimulates conversion of storage polymers in cereal grains.
Practical application of gibberellins
1. Germination : Gibberellin has been widely used for germination experiment. GA increases the length of hypocotyl and cotyledonary leaf area of Phaseolus aureus.
2. Rooting : Experimentally it has been suggested that gibberellins have no effect on rooting. Kato in 1958 found that not only root initiation is inhibited but that the stimulation of rooting caused by auxin is also counteracted.
3. Leaf expansion : In pea, bean, tomato, sweet corn, lettuce plants when treated with GA, leaves become broader and elongated.
4. Hyponasty of leaves : The GA treated Chrysanthemum plants hold their leaves more erect.
5. Flowering : GA induces flowering in long day plants and in plants requiring inductive cold period.
6. Parthenocarpy : Mukherjee and Dutta in 1962 observed that GA induces parthenocarpic fruits in case of brinjal.
7. Fruit setting : GA affects positively on fruit setting.
8. Fruit drop : The effect of GA on fruit drops is not clear.
9. Stem elongation : The most important effect of GA is the stem elongation.
10. Pollen germination : Ethirajan et al in 1963 found the effect of GA on pollen germination of sugarcane in vitro and found that out of 34, 15 varieties could be germinated by treating them with GA.
11. Breaking of dormancy : Potato tubers is made to germinate in winter by GA treatment.
Cytokinins
The term cytokinin was proposed by Letham in 1963. Cytokinins have been extracted from coconut milk, tomato juice, flowers and fruits of Pyrus malus etc. Letham obtained from corn grains. According to Skoog and Armstrong (1960) cytokinis of seven kinds from plants were obtained.
Applications of cytokinins
1. Cell division : Skoog in 1956 showed that IAA alone produces a few mitosis in tobacco pith tissues but kinetin in combination with IAA induced many mitosis.
2. Cell enlargement : The cell enlargement in the dices of etiolated leaf by cytokinins have been reported.
3. Morphogenesis : Skoog and Miller in 1957 found a response of cytokinins in the formation of organs. In case of tobacco pith, a balanced level of IAA and kinetin produces an amorphous and undifferentiated callus.
4. Dormancy : Cytokinins have been found very effective in breaking the dormancy of seeds and some other plant organs.
5. Apical dominance : Cytokinins counteract the usual apical dominance of bud. Cytoknins have been reported to induce lateral bud formation which has been found apparently related to the vascular tissue differentiation.
6. Initiation of interfascicular cambium : Kinetin can induce formation of interfascicular cambium.
7. Richmond-Land effect : In this case, kinetin was able to postpone for a number of days, the disappearance of chlorophyll and degradation of portiens which normally occur with the ageing process of leaves.
8. Mobility : Cytokinins are almost immobile in plants.
9. Nucleic acid metabolism : Guttman (1957) found a quick increase in the amount of RNA in the nuclei of onion root after kinetin treatment.
10. Protein synthesis : Osborne (1962) showed the increased rate of protein synthesis on kinetin treatment.
11. Incorporation in RNA : Plant tissue requiring cytokinins incorporate them into their RNA.
Abscisic acid (ABA) : Cairns and Addicott (1965) prepared two substances Abscisin-I From old cotton balls and Abscisin-II from young cotton balls which accelerated abscission of leaves.
Chemical nature of abscisic acid : Abscisic acid in short is called ABA. It possesses an asymmetric carbon atom and can, therefore, exist as either (+) or (–) enantiomers.
Translocation of abscisic acid : Analytical observations revealed that ABA moves through phloem and xylem.
Role of abscisic acid
1. Bud dormancy : It is believed that bud dormancy is controlled through changes in the levels of endogenous growth inhibitors (ABA).
2. Senescence : Senescence of leaf is promoted by ABA.
3. Abscission : ABA accelerated leaf abscission is cotton plants.
4. Flower initiation : In certain short day plants such as Ribes nigrum, Phorbitis and Fragaria, ABA produces flowering during long days.
5. Stomatal physiology : ABA alters the plasma membrane by affecting a change in a bioelectrical potential across them and an efflux of K+ ions.
6. Release of ethylene : It is observed that ABA stimulates release of ethylene.
7. Counteract GA : ABA Counteracts many effects of GA such as induction of hydrolases in barley seedlings.
Ethylene
Ethylene has tropistic responses of roots. Denny (1924) reported that ethylene is highly effective in inducing fruit ripening.
Characteristics of ethylene : Ethylene is a gas at temperatures under which a plant can live. It is a plant hormone. Maximum rate of ethylene production occurs during the period of maximum respiration just before senescence in many fleshy fruits.
Role of ethylene
1. Ethylene inhibits elongation of stem, causes swelling of node.
2. Ethylene retards flowering in most of the plants but accelerates in pineapple.
3. Ethylene increases the number of female flowers but decreases the number of male flowers.
4. Ethylene causes faster abscission of leaves and flowers.
Ethylene is a ripening agent. It is formed in large quantities in ripening fruits.
Few other growth hormones are morphactins, maleic hydrazide (MH), chlormequat, alar-85, amino-1618, phosphan D, caumarin, scopoletin and para-ascorbic acid.
Synthetic auxins : After the isolation of IAA as the natural auxin, a number of other chemical compounds have been synthesised which also exhibit auxin activity. These are called synthetic auxins.
Photoperiodism : The influence of the duration of daily periods of light and darkness on the phenomenon of flowering is called photoperiodism.
Period of day length for induction of flowering is classified into three groups—
(i) Short day plants (SDP) : These plants require a relatively short day light period (usually 8-10 hours) and continuous dark period of about 14-16 hours for flowering, e.g. Xanthium pensylvanicum, Glycine max, etc.
(ii) Long day plants (LDP) : These plants require a longer day light period (usually 14-16 hours) for flowering, e.g. Spinacea, Oleracea, Beta vulgaris, etc.
(iii) Day neutral plants : These plants flower in all photoperiod ranging from 5 hours to 24 hours continuous exposure, e.g. Mirabilis, Lycopersicum Esculentum etc.
(iv) Intermediate plants : Flower only in day lengths within a certain range usually 12-16 hours of light but fail to flower under either longer or shorter photoperiod, e.g. Andropogon furctus, Phaseolus polystachyus etc.
(v) Amphiphotoperiodic plants: Such plants remain vegetative on intermediate day lengths and flower only on shorter or longer day lengths, eg. Media elegans.
(vi) Short-long day plants : Flower when short photoperiods are followed by long photoperiods, e.g. some variety of Triticum vulgre, Secale cereale etc.
(vii) Long-short day plant : Flower when long photoperiods are followed by short photoperiods, e.g. Cestrum mocturnum, Bryophyllum, etc.
Most short day and long day plants have a definite critical photoperiod and specific influence of photoperiodic induction. For short day plants a definite continuous dark period is critical; so they are also called long night plants.
Phytochrome : Phytochrome is a bright blue or bluish green proteinaceous pigment found in two forms—
(i) Red light absorbing form designated as PR form.
(ii) Far red light absorbing form designated as PFR form.
PR and PFR are interconvertible, i.e. when PR form absorbs red light, it is converted into PFR form and when PFR form absorbs far red light, it gets converted into PR form.
660-665 mm
PR—PFR
730-735 mm
Besides photoperiodism, a large number of phytochrome mediated photorespones are known in plants, e.g. germination, sex expression, differentiation of stomata, increase in protein and RNA synthesis, auxins catabolism, epinasty, succulency, etc.
Photomorphogenesis : When plants are grown in continuous darkness they become weak i.e., such plants are longer, weaker, having yellowing 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 action spectrum of photomarphogenesis reveals that plants are most sensitive to red light, but blue light is ineffective.
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