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 natural growth inhibitors, 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 plant.
• 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 brings about an increase in ABA synthesis.

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

Ø Induces seeds to synthesize storage proteins.

Ø Inhibits the affect 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.