Availability of water for agriculture is being challenged increasingly because of a growing demand for water from other sectors such as industry, urban use, and for social and environmental purposes. The increase in demand for blue water (surface and sub soil) in future would perhaps be met at the cost of irrigation available to agriculture. Water is essential to plant growth as it provides the medium within which most cellular functions takes place. Water is also required as a unit of exchange for acquisition of CO 2 by plants. Water stress may conceivably arise either from an insufficient or from an excessive water activity in the plant's environment. In the case of terrestrial plants in nature, the former occurs as a result of a water deficit or drought and therefore is called a water deficit stress (shortened to water stress) or drought stress. Many physiological characteristics are correlated with the water potential of mesophyll tissue but the correlations are species specific. There is a general hierarchy of sensitivities among general physiological activities. Most sensitive are cell expansion, cell wall synthesis, protochlorophyll formation, and nitrate reduction. Generally turgor pressure is still accepted as the best indicator of water stress in plants. The specific mechanism by which turgor regulates physiological function probably relates to cell walls and membranes. Since cell expansion is dependent on cell pressure and the cell wall yield threshold, there can be no cell expansion without turgor pressure greater than the yield threshold for cell expansion. Studies with algal systems have indicated that slight changes in turgor pressure decrease membrane permeability to water and ions. Cell membrane structure and spatial arrangement of enzyme, transport channels, cellulose synthesis rosettes, and receptor proteins may be dependent on turgor pressure. Thus, when turgor pressure decreases, the spatial relationships of these proteins change, and membrane function is disrupted.Due to the wide variation in ambient temperature among environments where plants reside and the poikilothermic nature of plants, it is logical to expect a wide range of metabolic, morphological and anatomical adaptations to thermal