Type’s Growth Hormones and Its Effects
Most of the physiological activities and growth in plants are regulated by the action and interaction of some chemical substances called hormones.
In plants, they are termed as phytohormones. Some of the natural growth hormones in plants are as follows:
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1. Auxins:
Auxins were the first hormones to be discovered in plants and are universally present. They are nonspecific in their action, i.e., same auxin which influences growth phenomenon in one species also influences the same phenomenon in all species.
Physiological Effects
(i) Cell Elongation:
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Auxins stimulate the elongation of cells in shoot.
(ii) Apical Dominance:
The phenomenon in which the apical bud dominates over the lateral buds and does not allow the latter to grow is called apical dominance. This dominance of apical bud is under control of auxin which is produced at the terminal bud and transported downward through stem to lateral buds and thus hinders their growth.
(iii) Root Initiation:
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The higher concentration of auxin inhibits the elongation of roots, but the number of lateral branches of root is increased.
(iv) Prevention of Abscission:
Natural auxins have controlling influence on the abscission of leaves, fruits, etc.
(v) Parthenocarpy:
Auxins can induce the formation of parthenocarpic fruits.
(vi) Respiration:
Auxins stimulate respiration.
(vii) Callus Formation:
Auxins may be active in cell division. Callus (undifferentiated mass of cells) growth takes place only after addition of auxins in invitro tissue culture.
2. Gibberellins :
Yabuta and Sumuki (1938) isolated this hormone from the fungus Gibberella fujikuroi which was the causative agent of Backanae disease in rice seedling. It has now been isolated from a wide variety of higher plants.
Physiological effects
(i) Seed Germination:
Certain light sensitive seeds such as lettuce and tobacco show vigorous germination if the seeds are treated with gibberellic acid in the dark.
(ii) Dormancy of Buds:
Dormancy of buds in plants of temperate region can be broken by gibberellin treatment. In potato, the dormancy period can be overcome by application of gibberellin.
(iii) Root Growth:
Gibberellin has little or no effect on root growth.
(iv) Elongation of Internodes:
The most pronounced effect of gibberellin on plant growth is the elongation of internodes. Genetic dwarfism in pea and maize can be removed by the application of gibberellin.
(v) Parthenocarpy:
Growth of fruits and formation of parthenocarpic fruits can be induced by gibberellin treatment. Seedless and fleshy tomatoes and large sized grapes are produced by gibberellin treatment.
3. Kinetin (Cytokinins)
Kinetin hormone has not been isolated from any plant till now. Certain substances which show kinetin like activity have been isolated from a variety of higher plants. Some of the very important substances are:
(i) Coconut milk (liquid endosperm of coconut)
(ii) Zeatin (isolated from immature corn grains)
(iii) Degraded samples of yeast DNA.
Physiological Effects
(i) Cell Division:
It induces cell division in plants in the presence of auxin (IAA), especially in tobacco, carrot root tissue, soya bean and pea.
(ii) Cell Enlargement:
Like auxin and gibberellin, it may also induce cell enlargement.
(iii) Morphogenesis:
It has the ability to cause morphogenetic changes in callus (undifferentiated mass of cells). The tobacco pith callus can be made to develop either bud or root by changing the concentration of kinetin and auxin.
(iv) Counteraction of Apical Dominance:
Addition of kinetin along with auxin (IAA) stimulates the growth of lateral bud which otherwise is dominated by apical bud.
(v) Dormancy of Seeds:
Like gibberellin, kinetin can also break the dormancy of light sensitive seeds such as lettuce and tobacco.
(vi) Delay of Senescence:
Senescence of leaves could be postponed to several days by kinetin treatment.
4. Ethylene:
Ethylene is the smallest naturally occurring hormone in plants. Its recognition as natural plant hormone was done by Pratt Goeschl (1969).
Physiological Effects:
(i) It hastens post-harvest ripening of fleshy fruits like banana, pear, apple, tomato, etc.
(ii) It stimulates senescence and abscission of leaves.
(iii) It is effective in inducing flowering in pineapple plants,
(iv) It causes inhibition of root growth.
(v) It stimulates formation of adventitious roots.
5. Traumatic acid (wound hormone) :
Wound hormone is released by injured cells and causes the adjacent uninjured cells to become meristematic and divide till the wound is healed up.
6. Formative Hormone (Calines):
Formative hormones are thought to be essential for the effect of auxin on root, stem and leaf growth. They are of three types:
(i) Rhizocaline – Root forming hormone
(ii) Caulocaline- Stem forming hormone
(iii) Phyllocaline – Leaf forming hormone
7. Abscisic acid (ABA) :
Auxins, gibberellins and cytokinins are usually termed as growth promoters. Physiologists were aware that plant growth requires promotion as well as inhibition. They discovered it in the 1960s and named it Abscisic Acid (ABA).
It inhibits mitosis in vascular cambium and causes active axillary buds to become dormant with the approach of winter. ABA is also involved in the dormancy of seeds. Dormant seeds germinate when ABA is overcome by gibberellins.