ABSTRACT
The distribution of plants and animals over the surface of the earth is controlled more by the availability of water than by any other environmental factor. Even in humid regions such as western Europe and the southeastern U.S. crop yields are often reduced by drought. Unfortunately, it is difficult to establish a simple relationship between rainfall or soil water content and crop yield because plant processes are controlled directly by plant water status and only indirectly by soil water status. Plant water status is controlled by relative rates of absorption and transpiration; water deficits can result from either inadequate absorption caused by drying soil, excessive loss in transpiration, or from a combination of the two. On hot sunny days, water deficits may even develop in plants growing in moist soil; but during cool cloudy weather, plants growing in soil appreciably drier than field capacity may suffer only moderate deficits. 1 Furthermore, effects of water stress differ at various stages of growth and are often most severe when plants are in the reproductive stage, 2,18 as shown in Table 1.
