Abscisic Acid : Discovery, Occurrence and its Functions - Practice Questions & MCQ

• During the mid-1960s, three independent researchers reported the purification and chemical characterisation of three different kinds of inhibitors: inhibitor-B, abscission II and dormin. 
• Later all three were proved to be chemically identical. It was named abscisic acid (ABA).
• Abscisic acid is a mildly acidic dextrorotatory cis sesquiterpene growth hormone which functions as a general growth inhibitor by counteracting other hormones (auxin, gibberellins, and cytokinins) or reactions mediated by them.
• It is also called the stress hormone because the produc­tion of hormone is stimulated by drought, water logging and other adverse environmental conditions.
• ABA is synthesized in many parts of the plants but abundantly inside the chloroplasts of green cells.
• The hormone is formed from mevalonic acid or xanthophyll. 
• It is transported to all parts of the plant through diffusion as well as transport channels (phloem and xylem).
   

1. Bud Dormancy: Abscisic acid induces dormancy of buds towards the approach of winter.

2. Seed Dormancy: Dormancy allows seeds to tolerate desiccation and extremes of temperature better. The buds as well as seeds sprout only when abscisic acid is overcome by gibberellins. Because of its action in inducing dormancy, abscisic acid or ABA is also named as dormin.

3. Stoppage of Cambium Activity: Formation of abscisic acid stops mitosis in vascu­lar cambium towards the approach of winter.

4. Abscission and senescence: Abscisic acid promotes abscission of flowers and fruits. Its excessive presence stops protein and RNA synthesis in the leaves and hence stimulates their senescence (leaf fall is actually promoted by ethylene).

5. Transpiration: During desiccation and other stresses, abscisic acid is rapidly synthesised. The inhibitor causes closure of stomata and hence prevents transpiration.

6. Resistance: Abscisic acid increases resistance of plants to cold and other types of stresses. It is, therefore, also known as stress hormone.

7. Prevent starch hydrolysis: Abscisic acid inhibits gibberellin mediated amylase formation during germination of cereal grains.