Plant hormones are organic molecules that are produced at extremely low concentration in plants. These molecules control morphological, physiological and biochemical responses.
There are five major classes of plant hormones. They are:
4. Abscisic Acid (ABA)
Among all these plant hormones auxins, cytokinins and gibberellins promote plant growth while abscisic acid and ethylene inhibit plant growth.
Auxins (Gk. auxein = to grow) were the first plant hormones discovered. The term auxin was introduced by Kogl and Haagen-Smith (1931) . Auxins are produced at the tip of stems and roots from where they migrate to the zone of elongation. Charles Darwin (1880), observed unilateral growth and curvature of canary grass (Phalaris canariensis) coleoptiles. He came to the conclusion that some ‘influence’ was transmitted from the tip of the coleoptile to the basal region. This ‘influence’ was later identified as Auxin by Went.
Frits Warmolt Went (1903– 1990), a Dutch biologist demonstrated the existence and effect of auxin in plants. He did a series of experiments in Avena coleoptiles.
In his first experiment he removed the tips of Avena coleoptiles. The cut tips did not grow indicating that the tips produced something essential for growth.In his second experiment he placed the agar blocks on the decapitated coleoptile tips. The coleoptile tips did not show any response. In his next experiment he placed the detached coleoptile tips on agar blocks. After an hour, he discarded the tips and placed this agar block on the decapitated coleoptile. It grew straight up indicating that some chemical had diffused from the cut coleoptile tips into the agar block which stimulated the growth.
From his experiments Went concluded that a chemical diffusing from the tip of coleoptiles was responsible for growth, and he named it as “Auxin” meaning ‘to grow”.
Auxins are classified into two types, namely natural auxins and synthetic auxins.
1. Natural Auxins: Auxins produced by the plants are called natural auxins. Example: IAA (Indole – 3 - Acetic Acid)
2. Synthetic Auxins: Artificially synthesized auxins that have properties like auxins are called as synthetic auxins. Example: 2, 4 D (2,4 Dichlorophenoxy Acetic Acid).
Physiological effects of auxins: Auxins bring about a variety of physiological effects in different parts of the plant body.
1. Auxins promote the elongation of stems and coleoptiles which makes them to grow.
2. Auxins induce root formation at low concentration and inhibit it at higher concentration.
3. The auxins produced by the apical buds suppress growth of lateral buds. This is called apical dominance.
4. Seedless fruits without fertilization are induced by the external application of auxins. (Parthenocarpy). Examples: Watermelon, Grapes, Lime etc.
5. Auxins prevent the formation of abscission layer.
Cytokinins (Cytos - cell; kinesis - division) are the plant hormones that promote cell division or cytokinesis in plant cells. It was first isolated from Herring fish sperm. Zeatin was the cytokinin isolated from Zea mays. Cytokinin is found abundantly in liquid endosperm of coconut.
1. Cytokinin induces cell division (cytokinesis) in the presence of auxins.
2. Cytokinin also causes cell enlargement.
2. Both auxins and cytokinins are essential for the formation of new organs from the callus in tissue culture (Morphogenesis).
4. Cytokinins promote the growth of lateral buds even in the presence of apical bud.
5. Application of cytokinin delays the process of ageing in plants. This is called Richmond Lang effect.
Gibberellins are the most abundantly found plant hormones. Kurosawa (1926) observed Bakanae disease or foolish seedling disease in rice crops. This internodal elongation in rice was caused by fungus Gibberella fujikuroi. The active substance was identified as Gibberellic acid.
1. Application of gibberellins on plants stimulate extraordinary elongation of internode. e.g. Corn and Pea.
2. Treatment of rosette plants with gibberellin induces sudden shoot elongation followed by flowering. This is called bolting.
3. Gibberellins promote the production of male flowers in monoecious plants (Cucurbits).
4. Gibberellins break dormancy of potato tubers.
5. Gibberellins are efficient than auxins in inducing the formation of seedless fruit - Parthenocarpic fruits (Development of fruits without fertilization) e.g. Tomato.
Abscisic acid (ABA) is a growth inhibitor which regulates abscission and dormancy. It increases tolerance of plants to various kinds of stress. So, it is also called as stress hormone. It is found in the chloroplast of plants.
1. ABA promotes the process of abscission (separation of leaves, flowers and fruits from the branch).
2. During water stress and drought conditions ABA causes stomatal closure.
3. ABA promotes senescence in leaves by causing loss of chlorophyll.
4. ABA induces bud dormancy towards the approach of winter in trees like birch.
5. ABA is a powerful inhibitor of lateral bud growth in tomato.
Ethylene is a gaseous plant hormone . It is a growth inhibitor. It is mainly concerned with maturation and ripening of fruits. Maximum synthesis of ethylene occurs during ripening of fruits like apples, bananas and melons
1. Ethylene promotes the ripening of fruits. e.g. Tomato, Apple, Mango, Banana, etc.
2. Ethylene inhibits the elongation of stem and root in dicots.
3. Ethylene hastens the senescence of leaves and flowers.
4. Ethylene stimulates formation of abscission zone in leaves, flowers and fruits. This leads to premature shedding.
5. Ethylene breaks the dormancy of buds, seeds and storage organs.