Classes of hormones

a) Auxin b)Gibberelins c) Cytokinin    d) Abscisic acid     e) Ethylene

Function of Auxin

• Auxin stimulates cell elongation by inducing a localized H⁺ secretion from cells.

• Auxin activates massive synthesis of protein and RNA and cell division is initiated.

Function of gibberellic acid

• Gibberellic acid induces enzyme synthesis and causes cell differentiation.

Function of Cytokinin

• Inhibit blood platelet aggregation.
• Inhibit the growth of cells of some tumors.
• Stimulate elongation of cultured fibroblasts.t
• Induce remissions in leukemia patients.

Movement of cytokinin in plants

• Move to root tips, leaves, and probably the rest of the plant via xylem sap.

Metabolism of cytokinin

• Cytokinin is biosynthesized in root tips and developing buds, developing seeds.

Physiological effects of cytokinin

• Promotion of cell division and differentiation
• Retardation of senescence
• Promotion of germination of some seeds.
• Induction of tuberization in potato stolon
• Promotion of bud development.

Abscisic acid (ABA)

• Accelerate leaf abscission
• Promote bud dormancy in woody plants

Physiological effects of ABA

• Promote abscission in explants of cotton seedlings
• Induce bud dormancy
• Leaf senescence and fruit ripening

Biochemical Mode of Action of ABA

• Inhibit the efflux of ions from the guard cells and the flux of ions into them.

Physiological effects of ethylene

• Stimulation of ripening of fleshy fruits
• Abscission of leaves
• Inhibition of stem elongation
• Stimulation of radial swelling of stems
• Inhibition of root growth
• Stimulation of flowering
• Stimulation of leaf epinasty

Changes in ripening

• Tissue ripening
• Hydrolysis of storage compounds
• Changes in pigmentation and flavor
• Changes in respiration rate

Biochemical mode of Action of ethylene

• Ethylene stimulates the Lenovo synthesis and secretion of cell wall dissolving enzymes causing tissue softening.