Abscisic acid (ABA) is often referred to as a inhibitory rather than stimulatory hormone. It is involved in the closure of stomata, bud and seed dormancy and is known to inhibit other hormonal actions.
Discovery: F.T. Addicott and his associates discovered abscisic acid in the early 1960s in the process of studying abscission in cotton ( commercially important for mechanization of cotton picking') . It was also being studied by other plant physiologists at the same time for it's property of controlling abscission of flowers and in the initiation of dormancy of wood production.
It is found in leaves ( where it is partially synthesized), stems, and green fruits. and its biosynthesis is related to the process of cartenoid production. It is generally associated with negative-feedback interactions or stress-related environmental signals such as drought, freezing temperatures and environmental pollutants.
Effects of Abscisic Acid:
- Closure of Stomata -- Large amounts of abscisic acid in the leaves causes the stomata to close which helps the plant conserve water during droughts. Reactions can be instigated within minutes of spraying... commercially it is used in fields when droughts threaten.
- Bud Dormancy -- This is influenced by cytokinins and auxin-induced synthesis of ethylene.
- Seed Dormancy -- Abscisic acid delays seed germination in many plants. Has some effect on induction and maintenance of dormancy in general
- Induces seeds to synthesize storage proteins
- Inhibits the affect of gibberellins on stimulating de novo synthesis of a-amylase
It impact decreases when seeds germinate. In desert plants, ABA found in the outer layer of the seeds must be washed away by rains in order for the seeds to germinate.
- Counteracts Stimulatory Effects -- Abscisic acid inhibits stimulatory effects of other hormones.
- Inhibits shoot growth but will not have as much affect on roots or may even promote growth of roots. It inhibits cell division in the vascular cambium.
- Induces gene transcription especially for proteinase inhibitors in response to wounding (role in pathogen defense?).