Physiological mechanisms of water use efficiency in grapevines under drought conditions

Soil drought triggers a variety of physiological responses in plants, which may, directly or indirectly, influence water use efficiency. There is no unique definition of water use efficiency (WUE), but generally WUE is an indicator of the ratio of carbon acquired per unit water lost. Since plants are integrated units, stress responses leading to changes in WUE cannot be fully evaluated except in the context of the whole plant, taking under consideration adjustments occurring on different time scales and over a multitude of organization levels from biochemistry to leaf and canopy morphology. In grapevine, these adjustments and the degree of the responses vary with cultivar and the velocity of stress development, as well as the acclimation status of the plant. Field data are presented on a range of physiological mechanisms developing under drought conditions and how these mechanisms can act on WUE on the leaf and whole canopy level. It is shown that there are varietal differences in the capacity to elicit adaptive mechanisms to water deficit that can improve WUE, yet that improved WUE in the classical sense is not necessarily coupled to improved viticultural performance.