Plants produced certain chemical substances in their cells which regulate the growth of the plant body. These substances are formed in very minute quantities as a result of metabolic activities. They have a marked effect on certain physiological processes. Some of these chemicals simulate plant growth while some others retard the rate of growth of plants. These complex chemical substances are known as hormones.
A plant hormone is an organic substance which is produced in any part of the plant and is transferred or migrated to another part where it influences a specific physiological process.
Some of the plants hormones and their physiological effects are as follows:
1. Auxins
Auxins are the first group of plants hormones which are produced from the root and shoot-tips of the plant body. They were first discovered by the Dutch botanist F W went (1928) while experimenting on oat (Avena sativa). Apical meristems, young flower buds, young leaves and developing seeds are the sides of their production. After their production, 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.
Function of Auxins
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Auxins are known to take part in a number of plant processes. Auxins promote cell elongation, root formation, cell division, respiration and other physiological processes like protein synthesis, water uptake and protoplasmic permeability. Auxins also play an important role in the development of seedless fruits without pollination and fertilization. This phenomenon is called parthenocarpy.
2. Gibberellins
The discovery of gibberellins is connected with the bakanae disease of rice. It was first reported in Japan by E Kurosawa (1926). Gibberellins are growth hormones which promote cell elongation.
Functions of Gibberellins:
Gibberellin stimulates stem elongation, seed germination and flowering. It is about 500 times more effective than auxin in inducing parthenocarpy in tomato. It counteracts the effect of dormin (abscisic acid) which caused dormancy or inhibits growth.
3. Cytokinins or Kinins
Cytokinins are the chemicals which promote cytokinesis in cells of various plant origins. They are synthesized in the endosperm of the seeds and roots of the plants. The first cytokinin was isolated by Miller et al. (1954). Zeatin was the first natural cytokinin of plants.
Functions of Cytokinins:
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Cytokinis are essential for the division of cytoplasm i.e. cytokinesis. Like gibberellins, cytokines also help in breaking dormancy. They also regulate phloem transport.
4. Ethylene
Ethylene is a gaseous hormone 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.
Functions of Ethylene: Ethylene is helpful in ripening fruit. It breaks the dormancy of several plant organs. It induces flowering growth of roots and root hairs.
5 Abscisic Acid or Abscisin (ABA)
Abscisic acid is a growth inhibitor which reverses the promontory effects of auxins and gibberellins.
Functions of ABA:
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It is a general inhibitor of growth and in this way its function is opposite to those of auxins and cytokinins. It causes dormancy of seeds, tubers and bulbs. It promotes leaf and fruit fall. It helps in the closure of stomata to decrease the loss of water. It is responsible for the loss of RNA, proteins and chlorophylls.