Phytohormones Cytokinin

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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 theendosperm 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.

Physiological effects of cytokinin

The most important function of cytokinin is the promotion of cell
division.
In association with IAA, cytokinin initiates bud and root formation in callus tissue.
External application of cytokinin promotes the growth of lateral buds even if the apical bud is intact.
Cytokinin breaks the dormancy of many seeds and also promotes
germination.
Application of cytokinin delays the process of ageing in plants. This is also known as Richmond Lang effect.

Ethylene

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.

Physiological effects of ethylene

Ethylene prevents elongation of stem and root in longitudinal direction. Simultaneously, the tissue enlarges radially resulting in thickening of plant parts.

Ethylene promotes positive geotropic growth of roots.
A ethylene inhibits the growth of lateral buds in pea seedlings.
Ethylene is involved in the ripening of fruits.
The ethylene stimulates the formation of abscission zone in leaves, flower sand fruits. This causes leaves, flowers and fruits to shed prematurely.
Flowering can be induced by application of ethylene in plants like
pineapple and mango.
Ethylene stimulates rooting of cuttings, initiation of lateral roots and growth of root hair.
The ethylene is responsible for breaking the dormancy of buds and seeds.

Abscisic acid

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.

Physiological effects of abscisic acid

Abscisic acid acts as growth inhibitor and induces bud dormancy in a variety of plants.
ABA is a powerful growth inhibitor. It causes 50 per cent inhibition
of growth of oat seedlings.
As the name suggests abscisic acid is an hormone that stimulates
abscission.
ABA controls geotropic responses of roots. It stimulates positive
geotropism in roots.
Abscisic acid causes closure of stomata.

Growth inhibitors

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.

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Other links

Plant tissue culture – origin and techniques

Plant physiology – photosynthesis and its significance

Site of photosynthesis and Mechanism of photosynthesis

Electron transport system and photophosphorylation types

Dark reaction

C3 and C4 pathways

Photorespiration or C2 cycle

Factors affecting photosynthesis

Test tube and funnel experiment, Ganong’s light screen experiment

Mode of nutrition – Autotrophic, Heterotrophic