Introduction

On a global basis, drought limits plant growth and crop productivity more than any other single environmental factor (Boyer, 1982). Even in Britain, rain-free periods are frequent enough for irrigation to lead to yield advantages for many agricultural and horticultural crops. Water supply is restricted in many parts of the world and productivity in these environments can only be increased by the development of crops that are well adapted to dry conditions. It is clear that the potential for biotechnological improvement of crop performance cannot be realised until we have identified genes and gene products which are responsible for the desired characteristics of drought tolerance. This in turn cannot occur without a thorough understanding of the biophysical, biochemical and physiological perturbations that are induced by a restricted water supply.

Although plant growth rates are generally reduced when soil water supply is limited, shoot growth is often more inhibited than root growth and in some cases the absolute root biomass of plants in drying soil may increase relative to that of well-watered controls (Sharp & Davies, 1979; Malik, Dhankar & Turner, 1979). It is also commonly observed (e.g. Sharp & Davies, 1985) that the roots of unwatered plants grow deeper into the soil than roots of plants that are watered regularly. Clearly, increases in the density and depth of rooting can help sustain a high rate of water extraction in drying soil (Sharp & Davies, 1985) and may promote substantial improvement in yield in dry years (Jordan, Dugas & Shouse, 1983).