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The overall effects of water deficits on crop yield are known to everybody. Moderate water deficiencies can result in stunting, distorted development, and much reduced crop yields, and prolonged drought can cause complete crop failure.
Despite these visible symptoms of deficiency, a clear and unambiguous statement as to the effect of deficits on crop yield is difficult to make, for a variety of reasons. Firstly, despite the importance of the phenomenon to agriculture in most of the food producing countries of the world, relatively little attention has been devoted to its water deficits. Secondly, and in part this is a reason for the relative lack of effort ,plant water status is a highly dynamic parameter, strongly influenced by conditions in the soil and atmospheric microenvironment, and also regulated to different degrees in different situations and with different species, by physiological factors. It therefore constitutes a difficult parameter to examine experimentally. Thirdly, as is the case with most types of lesion, water deficits affect the growth and development of crops in many ways, both directly and indirectly. Consequently, it is frequently difficult to assign cause and effect relationships with confidence.