"Review of literaturePlants in their natural environment are continuously exposed to different typesof biotic and abiotic stresses which consequently affect their productivity (Seki et al.,2003). Water stress is the major limiting factor in the world for agriculture produce. Drought stress accounts for about 30 to 70 percent of crop yield loss worldwide.Drought stress occurs when the available water in the soil is insufficient to meet thecontinuous loss of water by transpiration.Plant responses and adaptation to drought stressPlants have developed various strategies to cope with the physiological eventsassociated with drought. These fall into three categories, drought escape, avoidance andtolerance. EscapeDrought escape refers to the growth patterns that plants have developedthroughout evolution to reduce their exposure to environmental stress. This aspect ofdrought resistance includes the development of leaves that are small and thick withwaxy cuticles, and the leaves with a higher concentration of stomata on the lower side(Chaves et al., 2003). In arid regions, escape strategies help the plants to complete theirreproductive cycle before the onset of drought stress by using the available moisture7 Review of literaturewith high rates of growth and gas exchange (Ehleringer and cooper, 1992; Maroco etal., 2000).Avoidance Drought avoidance refers to the strategies adopted by plants to maintain highwater potential during periods of water deficit. Water loss is minimized by closingstomata, by reducing light absorbance through rolled leaves (Ehleringer and Cooper,1992), a dense trichome layer increasing reflectance (Larcher, 2000) or steep leafangles or by decreasing canopy leaf area through reduced growth and shedding of olderleaves. The pattern of root growth also changes in response to water availability. Themore elaborate the root system, greater the chance that plant will survive during dryspells. In groundnut genotypic variations in root depth, root volume and waterextraction pattern at different depths have been reported (Ketring, 1984).ToleranceDrought tolerance involves mechanisms that allow plants to maintain theirmetabolism and limit the harm caused by prolonged drought (Courtois et al., 2000).Drought tolerance capacity of the plant differs within the species and between thespecies. The tolerance capacity varies depending on developmental stage, tissue ororgan type and with seeds being the most tolerant tissue (Umezawa et al., 2006). Plantshave developed different kinds of strategies to survive the events associated withdrought tolerance (Courtois et al., 2000) (Figure 1). Tolerance to low tissue waterpotential may involve osmotic adjustment (Morgan, 1984) more rigid cell walls orsmaller cells (Wilson et al., 1980). Evergreen shrubs and trees in the arid or semi-aridregion maintain high solute concentration in tissues (low osmotic potential) combinedwith low photosynthetic rate because of low stomatal conductance as a tolerancemechanism to survive (Jarvis and Mcnaughton 1986). Development of small leaves in8 Review of literaturethe majority of the arid regions is also an important tolerance strategy since smallleaves permit greater sensible heat loss and controls water loss by the stomatal closure(Ingram and Bartels, 1996).Plant responses to drought stress may be classified as i) short-term changesrelated to mainly physiological responses (linked to stomatal regulation) ii) acclimationto availability of certain level of water (solute accumulation resulted with adjustment ofosmotic potential and morphological changes) iii) adaptation to water stress conditions(physiological mechanisms and modifications in anatomy) (Kozlowski and Pallardy,2002). Jones et al., (1980) stated that the rate of development of stress has a majoreffect on the degree of osmotic adjustment. Oosterhuis and Wullschleger (1987)pointed out that increasing the number of stress cycles increased the amount of osmoticadjustment in cotton. Turner (1986) noted that leaf expansion can decrease without achange in leaf turgor pressure. The osmotic adjustment has been found in many species(Morgan, 1984) and has been implicated in the maintenance of stomatal conductance,photosynthesis, leaf water volume and growth (Medrano et al., 2002).Morphological responses of plants to drought stressDrought stress is a very important limiting factor at the initial phase of plantgrowth and establishment. Drought stress affects cell elongation, cell expansion, andcell growth (Bhatt and Srinivasarao, 2005; Kusaka et al., 2005; Shao et al., 2008). Insoybean, the shoot length was decreased under water deficit conditions (Specht et al.,2001). The plant height was reduced up to 25% in water stressed citrus seedlings (Wuet al., 2008). Stem length was significantly affected by water stress in potato (Nadlerand Heuer, 1995), Abelmoschus esculentus (Sankar et al., 2007 and 2008); Vignaunguiculata (Manivannan et al., 2007) soybean (Zhang et al., 2004) and parsley(Petroselinum crispum) (Petropoulos et al., 2008). The reduction in plant height was9 "