Variability in adaptive mechanisms to water deficits in annual and perennial crop plants

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Abstract
Terrestrial plants have evolved numerous mechanisms which enable them not only to survive, but to achieve substantial productivity levels under drought. Most adaptive mechanisms appear to be directed at either avoiding or tolerating plant water deficits. The sensitivity of leaf area expansion to mild water stress is emphasized, suggesting that it has adaptive significance by acting as a partitioning mechanism which favours root growth and hence improves the shoot water status. Alterations in developmental pathways often aid the plant in avoiding dehydration by adjusting its life cycle to the available water supply. Mechanisms of dehydration tolerance include the control of transpiration and the maintenance of turgor through osmotic adjustment. Examples of adaptive mechanisms and their possible role in yield responses under water stress are presented for sorghum, cotton and sunflower as annuals and orange, almond and olive trees as perennials. There is substantial variability in the degree of expression of several adaptive mechanisms and this has important implications for the improvement of crop production under water stress.