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Plant water deficits usually result in severe inhibition of shoot growth, while root growth is less inhibited or even promoted. Recent advances in understanding the physiology of the differential responses of root and shoot growth to low water potentials will be reviewed. While it might be readily accepted that hormones arc important in transducing environmental conditions into growth responses, there is surprisingly little definitive evidence for the role of any hormone in regulating plant growth in soils of low water potential. Using maize seedlings as a model system, the increase in ABA that accompanies plant water deficits has been shown to be required for root growth maintenance, and also to play a role in shoot growth inhibition. The action of ABA in root growth maintenance appears to involve regulation of ethylene synthesis and/or sensitivity, while the mechanism of shoot growth inhibition is not known. Evidence that ABA acts as a root `signal controlling shoot growth in drying soil will also be considered. The importance of osmotic adjustment as a mechanism of growth maintenance at low water potentials has been questioned by suggestions that solute accumulation may be merely a consequence of stress-induced growth inhibition. Recent studies will be discussed which do not support this idea, and suggest that the response may be useful for crop improvement.

991 REGULATION OF PLANT GROWTH RESPONSES TO LOW SOIL WATER POTENTIALS