The long-distance abscisic acid signal in the droughted plant: the fate of the hormone on its way from root to shoot.

Abscisic acid (ABA) is a potent molecule that certainly modifies stomatal behaviour and plant water loss and probably acts to modify the growth of leaves. The hormone is synthesized both in the leaves and the roots of the plant and in the soil and may move freely from plant to soil and soil to plant. It can also move rapidly through the plant in both the xylem and the phloem and will partition between different compartments in different tissues largely as a function of pH. It is described here how perturbations in soil conditions around the roots and the water status of the air can modify the fluxes of ABA around the plant and its accumulation in different compartments and different tissues. These fluxes can be interpreted as signals of different stresses imposed on the plant and consideration is given to how different perturbations can exert subtle changes which are manifest as modified shoot growth rates and functioning. Most emphasis in the discussion is placed upon the plant's responses to the imposition of soil and atmospheric drought.