It is the net result of various processes that together cause an irreversible permanent increase in the size of an organism or its parts. In plants, growth take place by cell division and cell enlargement.
The first step in the process of plant growth is seed germination, which needs favourable conditions for growth in the environment.
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.
Role of meristems in growth :
Meristems are divided into two main categories :
1. Apical or Terminal meristems : These are located near the tips of roots and stems and also the growing young leaves, near the tips of stems as well on the tips of axillary buds. As a result of the activity of these meristems plant increases in length.
2. Cambium or Lateral meristems: These are situated below the bark and is responsible for increase in the diameter of the stem.
CONDITIONS FOR GROWTH
The condition necessary for growth are similar to that of synthesis of protoplasm and cell division. The supply of nutrients, water, oxygen, suitable temperature and light are necessary for proper growth. The force of gravity and light also determines the direction of root and shoot growth.
PLANT GROWTH REGULATORS
It is an established fact that plants produce some specific chemical substances, which are capable of moving from one organ to the other, so as to produce their effect on growth.
These substances, which are active in very small amounts are called Plant hormones (or Phytohormones).
The plant hormones are organic substances which are produced in any part of the plant and is transferred or migrated to another part where it influences a specific physiological process, leading to promotion, inhibition and modification of growth.
These growth regulatory substances are broadly grouped under five major classes, namely Auxins, Gibberellins, Cytokinins, Abscisic acid and Ethylene.
Growth promoters and growth inhibitors
Based on their functions in a living plant body, plant growth regulators can be broadly divided into two groups: Growth promoters and growth inhibitors.
Some of these chemicals simulate plant growth while some others retard the rate of growth of plants. The growth promoters are : Auxins, Gibberellins, Cytokinins. The growth inhibitors are : Abscisic acid and Ethylene.
Discovery of Plant Growth Regulators (Plant Hormones)
In last 60 years, a large number of growth regulators have been isolated from plants and their action was studied. The first indication of its exitance was given by Darwin (1880), who was studying the bending of the coleoptile of a Canary grass towards light.
He found that light falling on the tip of grass coleoptile causes some influence. This influence (plant hormones) is transmitted downward and causes the coleoptile to bend towards light.
Darwin also demonstrated that when the coleoptile tip was removed it did not bend when exposed to light. The phototropic response did not occur.
Boysen – Jensen (1910 – 1913)
He demonstrated that insertion of mica plate on the shaded side of the coleoptile tip, prevented the bending of the tip towards light.
When the mica sheet was inserted towards illuminated side the coleoptile tip bent towards light in the usual way. Thus, it was concluded that a substance (plant hormones called auxins) migrates down the dark side promoting growth curvature towards light.
FUNCTIONS OF PLANT HORMONES (PHYTOHORMONES)
- Flowering of plants
- Ripening of fruits
- Germination of seeds
- Breaking dormancy of seeds
- Stomatal movement in leaves
- Phototropism, geotropism, chemotro- pism, hydrotropism, thigmotropism and nastic movements.
PHYSIOLOGICAL EFFECTS OF PLANT GROWTH REGULATORS (PLANT HORMONES)
Some of the plant hormones and their physiological effects are as follow:
Auxins (Gk. Auxein : to grow) isolated initially from the urine of humans suffering from Pellagra, whose role in humans is not known, were the first plant hormones to be discovered.
Auxins are the first group of plant hormones which are produced from the root and shoot-tips of the plant body, from where they migrate to the region of their action. They were first discovered by the Dutch Botanist FW Went (1928) while experimenting on oat plant (Avena sativa).
After their production from shoot tip, auxins migrate downwards and influence the zone of elongation. Similarly, from the root tip, auxins migrate upwards to the zone of elongation where they impart their effect. It was about Auxins Plant Hormones.
Types of Auxins
Auxins are of following two types :
A. Natural auxins: These plant hormones have been isolated from plants where they occur naturally. The best known examples of natural auxins are Indole acetic acid (IAA) and Indole butyric acid (IBA).
B. Synthetic auxins : A large number of synthetic plant hormones are presently being used in agriculture. The examples of synthetic auxins plant hormones are 2, 4 dichlorophenoxy acetic acid (2,4 -D) and Naphthalene acetic acid (NAA).
Physiological effects of auxins on plants:
Auxins control several kinds of plant growth processes. These are as follow :
- Cell elongation: Auxins promote elongation and growth of stems and roots. It also promotes enlargement of many fruits by stimulating elongation of cells in all directions.
- Apical dominance: In many plants, the apical bud grows and lower axillary buds are suppressed. Removal of apical buds results in the growth of lower buds. The auxins of the terminal buds inhibits the growth of lateral buds. This phenomenon is called Apical dominance.
- Control of Abscission layer : In plants like apple, pear and citrus, shedding of leaves, flowers and fruits take place due to the formation of a separation layer at the base of petiole or pedicel. If a dilute solution of auxins is sprayed upon the plant fruit fall may be delayed until they are fully matEred.
- Weed eontrol : Weeds are unwanted plants growing in a field alongwith a crop. These weeds cause competition for water, minerals, light and space and thus do not allow proper growth of the crop. By spraying auxins they can be destroyed and removed.
- Root inducing : Plants like rose and Bougainvillea are usually propagated by stem cuttings. If the lower cut ends of a cutting is dipped in auxins, very soon large number of roots are developed on the cut ends due to which these cuttings develop into successful plants.
- Parthenocarpy: It is the phenomenon of development of seedless fruit without fertilization. When a flower bud is emasculated and auxins applied to the stigma of the flower, a seedless fruit develops.
- Flowering : In pineapple, auxins promotes flowering. In lettuce, auxins help in delaying the flowering. In cotton plants, auxins increases the cotton seed production.
- Sex expression : The spray of auxins increases the number of female flowers in cucurbits. Thus, auxins causes femaleness in plants.
Gibberellins were first isolated from the fungus Gibberella fujikuroi, the causal organism of foolish seedling or bakanae disease of rice.
It wan first reported in Japan by E Kurosawa (1926), Presently more than 100 gibberellins have been identified. Unlike auxins, they enhance the longitudinal growth of stem when applied to the plants. Gibberellins are acidic in nature.
Physiological effects of Gibberellins on plants :
- Seed germination : Gibberellins promote seed germination in cereals and lettuce.
- Leaf expansion : In pea, bean, tomato and cabbage on gibberellins they develop broader and elongated leaves.
- Parthenocarpy: Gibberellins are more potent parthenocarpic agents than auxins. treatment with
- Stem elongation : Gibberellins causes stem elongation and leaf expansion but have no effect on roots. Pronounced elongation responses observed with certain genetic dwarf varities of pea, bean and maize while normal tall plants are not effected by gibberellins treatment.
- Bolting : In cabbage, internodes are reduced. Just prior to flowering internode elongates enormously. This is called Bolting. By application of gibberellins bolting can be induced artificially.
- Increasing fruit size : Gibberellins are used to increase fruit size and bunch in have been grapes.
- Breaking of dormancy : Gibberellins break dormancy of buds and tubers.
- Sex expression: In general, gibberellins promote production of male flowers in place of female flowers as in Cannabis.
Cytokinins are the plants hormones which promote cytokinesis in cells of various plant origins and prevent ageing in tissues. They have little or no effect on growth.
They are synthesized in the endosperm of the seeds and roots of the plants. Overbeek, a botanist reported the presence of cytokinins in coconut milk.
Miller isolated and identified a cytokinins from endosperm of corn and called it zeatin. It was about Cytokinins Plant Hormones.
Physiological effects of Cytokinins on plants:
- Cell division : In the presence of auxins, cytokinins stimulate cell division.
- Delay in ageing : Cytokinins delay ageing process in plant organs by controlling protein synthesis.
- Breaking dormancy of seeds : Cytokinins break the dormancy of many seeds and promote their germination.
- Accumulation and Translocation of solutes : Cytokinins help the plants in accumulation of salts within the cells These also help in the translocation of solutes in phloem.
- Resistance to high temperatures: Cytokinins provide resistance to plants injured by high or low temperatures.
- Morphogenesis : In tissue cultures of parenchyma, mitosis is accelerated when both auxins and cytokinins are present, but no response is seen with auxins or cytokinins present alone.
Ethylene is a simple gaseous plant hormones which stimulates transverse growth. This growth regulator is produced in the cells of higher plants, especially those under stress.
It is produced by fruits, flowers, seeds, leaves and roots. Its importance in growth of plants was first shown by Neljubov in 1901. Galston and Davies (1970) recognized it as a growth regulator.
Physiological effects of Ethylene on plants:
- Flowering : Ethylene plant hormones retard flowering in most plants but induces flowering in pineapple and mango.
- Sex modification : Application of ethylene increases the number of female lowers and fruits in cucumber plant.
- Fruit ripening : In banana, apple, mango and citrus ethylene stimulates ripening of fruits.
- Inhibitory effect on growth : Ethylene modifies growth by inhibiting the elongation of stems and roots. In stem, it causes swelling of nodes.
- Dormancy : Ethylene breaks dormancy of buds and seeds.
Abscisic Acid (ABA)
Abscisic acid is a natural growth inhibitor which retards growth. It has been isolated from several parts of higher plants. It is synthesized in leaves from where it is transported to other narts through phloem. In most situations it acts as an antagonist to gibberellins.
Physiological effects of Abscisic Acid on plants :
- Cell division and cell elongation: Abscisic acid inhibits cell division and cell elongation.
- Resistance to cold : Abscisic acid increases resistance in plants to cold.
- Antitranspirant : In plants subjected to salinity or under severe drought, abscisic acid reduces the rate of transpiration by temporarily closing the stoma.
- Bud dormancy : Abscisic acid induces bud dormancy in many plants. It causes active axillary buds to become dormant with the approach of winter in perennial plants.
- Rooting : In Ivy and bean, abscisic acid promotes rooting of stem cuttings.
- Triple response : Ethylene inhibits elongation of stem, causes swelling of nodes and nullifies geotropism.
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