Cytokinin is a plant growth substance, which stimulates cell division. This was isolated by Miller and Skoog in 1954 from Herring fish. Following the discovery of kinetin many other compounds showing similar activity were discovered. These are collectively called cytokinins. The cytokinin found in the zea mays is called zeatin. Cytokinin is also found in the endosperm of coconut. Cytokinin occurs in various seed plants. They are found particularly in embryos, developing fruits and roots. Varying mixtures of auxin and cytokinin influence plant growth and differentiation.
1. The most important function of cytokinin is the promotion of cell division.
2. In association with IAA, cytokinin initiates bud and root formation in callus tissue.
3. External application of cytokinin promotes the growth of lateral buds even if the apical bud is intact.
4. Cytokinin breaks the dormancy of many seeds and also promotes germination.
5. Application of cytokinin delays the process of ageing in plants. This is also known as Richmond Lang effect.
Ethylene is a simple gaseous hormone. It is usually present in a minute quantity. It is synthesised in large amounts by tissues undergoing ageing and acts as a natural plant growth hormone.
1. Ethylene prevents elongation of stem and root in longitudinal direction. Simultaneously, the tissue enlarges radially resulting in thickening of plant parts.
2. Ethylene promotes positive geotropic growth of roots.
3. Ethylene inhibits the growth of lateral buds in pea seedlings.
4. Ethylene is involved in the ripening of fruits.
5. Ethylene stimulates the formation of abscission zone in leaves, flowers and fruits. This causes leaves, flowers and fruits to shed prematurely.
6. Flowering can be induced by application of ethylene in plants like pineapple and mango.
7. Ethylene stimulates rooting of cuttings, initiation of lateral roots and growth of root hair.
Ethylene is responsible for breaking the dormancy of buds and seeds.
Abscisic acid (ABA) was originally discovered for its role in regulating abscission and bud dormancy. Like other plant hormones, it has multiple functions in the growth of plants.
1. Abscisic acid acts as growth inhibitor and induces bud dormancy in a variety of plants.
2. ABA is a powerful growth inhibitor. It causes 50 per cent inhibition of growth of oat seedlings.
3. As the name suggests abscisic acid is an hormone that stimulates abscission.
4. ABA controls geotropic responses of roots. It stimulates positive geotropism in roots.
5. Abscisic acid causes closure of stomata.
Some organic substances produced in the plant inhibit the plant growth. These substances are called growth inhibitors. They inhibit the elongation in roots, stems and leaves. For example, ethylene is a potent inhibitor of bud growth. ABA inhibits lateral bud growth in tomato.