Drought (water stress) is one the major abiotic stress factors that affect all organisms lives including human in terms of health and food. Water absence from the soil solutions affect the natural evaporative cycle between earth and atmosphere that contribute amount of rainfall. Drought occurs when soil moisture level and relative humidity in air is low while temperature is also high. UN reports (2006) estimate that one third of world population has been living in areas where the water sources are poor. Water stress resulting from the withholding of water, also changes the physical environment for plant growth as well as crop physiology. Almost every plant process is affected directly or indirectly by water supply. Plants, as one of basic food sources, either in nature or cultivations, in their growing period, require water or at least moisture for germination. Certainly, most land plants are exposed to short or long term water stress at some times in their life cycle and have tended to develop some adaptive mechanisms for adapting to changing environmental conditions. Some plants may adapt to changing environment more easily than others giving them an advantage over competitors. Water stress may range from moderate, and of short duration, to extremely severe and prolonged summer drought that has strongly influenced evolution and plant life. Crop yields are restricted by water shortages in many parts of the world. The physiological responses of plants to water stress and their relative importance for crop productivity vary with species, soil type, nutrients and climate. On a global basis, about one-third of potential arable land suffers from inadequate water supply, and the yields of much of the remainder are periodically reduced by drought. It is estimated that 10 billion people in the world will be hungry and malnourished by the end of this century. One of the aims of the researches is to gain an understanding of survival mechanisms which may...
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