The complex responses of stomata which provide protection for land plants against excessive water loss are best understood if we consider them as occupying two lines of defence. The first line of defence consists of immediate responses to factors of the aerial environment, especially carbon dioxide concentration and water vapour pressure deficit, which ensure that the rate of transpiration is regulated to a level which can be supported by water uptake through the roots in moist soil. When the soil becomes dry, further controls become necessary, and the second line of defence comes into operation. A ceiling is imposed on the extent to which stomata can open, and an increase in the efficiency of water use is achieved, though at the expense of some reduction in the rate of photosynthesis. A sesquiterpenoid, abscisic acid (ABA) plays a major part in the second line of defence. It is contained in the mesophyll chloroplasts in leaves of well watered plants and is released when the water potential falls; the synthesis of new ABA is also induced by water stress. Movement of ABA from the mesophyll to the guard cells is assumed to take place, because the chloroplasts of guard cells appear to be unable to form ABA in response to water stress. We suggest that farnesol, another sesquiterpenoid hitherto considered to have a separate role as a regulator of transpiration, is the agent responsible for altering the permeability of chloroplast envelope membranes, allowing the release of ABA into the cytoplasm. The closure of stomata induced by ABA appears to be part of a series of integrated responses throughout the plant which helps to maintain turgor and growth when water is in short supply.
Abscisic Acid Is Required for Root Elongation Associated With Ca2+ Influx in Response to Water Stress