Absicissic Acid (ABA)

• Functions by inhibiting growth activities in times of environmental stress rather than by promoting growth.
• It often serves as an antagonist to the other growth-promoting hormones in plants.
• It is a natural growth inhibitor and acts as a stress hormone that helps to cope with adverse environmental conditions.

Role of ABA

• Induces bud dormancy
• Promotes senescence
• Accelerates leaf abscission in cotton pplants
• Induces flowering during long days in certain short-day plants – ineffective in short-day plants.
• Counteracts GA

Physiological effects caused by cytokinins

Ø Stimulates the closure of stomata-water stress bringing about an increase in ABA synthesis.

Ø Inhibits shoot growth but will not have as much effect on roots or may even promote the growth of roots.

Ø Induces seeds to synthesize storage proteins.

Ø Inhibits the effect of gibberellins on stimulating.

Ø Has some effect on induction and maintenance of dormancy.

Ø Induces gene transcription, especially for proteinase inhibitors in response to wounding which may explain an apparent role in pathogen defense.