Growth and Development of Horticultural Crops (1+1)

Physiological role

1. Stomatal regulation

• The role of ABA in causing stomatal closure in plants undergoing water-stress is now widely recognized. It has been suggested by various workers that in response to the water-stress, the permeability of the chloroplast membranes of mesophyll cells to ABA is greatly increased.
  
• As a result, the ABA synthesized and stored in mesophyll chloroplasts diffuses out into the cytoplasm. It then moves from one mesophyll cell to another through plasmodesmata and finally reaches the guard cells where it causes closing of stomata. Fresh biosynthesis of ABA continues in mesophyll chloroplasts during periods of water stress.
  
• When water potential of the plant is restored (i.e., increased), the movement of ABA into the guard cells is arrested. ABA disappears from the guard cells a little later. The application of exogenous ABA causes closing of stomata by inhibiting the ATP-mediated H+/K+ ions exchange pumps in guard cells.
2. Leaf abscission

• ABA is known to produce abscission layers at the base of the leaf petiole where dead cells are formed. ABA production increases in senescing leaves once the photosynthetic activity of the leaves decreases below the compensation point.
3. Seed and bud dormancy

• Seeds and buds remain dormant to ward off unfavourable seasonal and soil conditions for germination and growth respectively. Presence of ABA in such seeds and buds provides dormancy to these structures. Once favourable conditions are available, ABA gets denatured or overcome by production of growth promoting hormones such as GA or IAA.
4. Other Functions

• Process of tuberization, fruit ripening, increasing the resistance of temperate zone plants to frost injury, inhibition of GA-induced synthesis of α-amylase in aleurone layers of germinating barley, inhibition of precocious germination and vivipary and increase in root: shoot ratio at low water potentials.