Plant Water Relations

Abstract

Although water is the most abundant molecule on the Earth’s surface, the availability of water is the factor that most strongly restricts terrestrial plant production on a global scale. Low water availability limits the productivity of many natural ecosystems at different time scales (Fig. 5.1). In addition, losses in crop yield due to water stress exceed losses due to all other biotic and environmental factors combined (Boyer 1985). Water availability is also a major determinant of plant and biome distribution. Regions where rainfall is abundant and fairly evenly distributed over the growing season, such as in the wet tropics, have lush vegetation. Where seasonal droughts are frequent and severe, forests are replaced by grasslands or savannas, as in the Asian steppes, North American prairies and tropical savannas (Hirota et al. 2011). Further decrease in rainfall results in semideserts, with scattered shrubs, and finally deserts. Even the effects of temperature are partly exerted through water relations, because rates of evaporation and transpiration are correlated with temperature. Thus, if we want to explain natural patterns of productivity or to increase crop productivity, it is crucial that we understand the controls over plant water relations and the consequences for plant growth of an inadequate water supply. Understanding plant water relations is also important to improve our ability to predict the effects of more frequent extreme climatic events such as droughts and floods on the future distribution and functioning of natural ecosystems.