Environmental stresses trigger a wide variety of plant responses, ranging from altered gene expression and cellular metabolism to changes in growth rate and crop yield (Bhatt et al. 2005; Jaleel et al., 2007a; Farooq et al. 2008). Plant reactions exist to circumvent the potentially harmful effects caused by a wide range of both abiotic and biotic stresses, including light, drought, salinity and high temperatures (Chaves et al. 2002; Jaleel et al., 2007b, 2008a,b ). Water stress is the major problem in agriculture and the ability to withstand such stress is of immense economic importance. Water stress tolerance involves subtle changes in cellular biochemistry. It appears to be the result of accumulation of compatible solutes and of specific proteins that can be rapidly induced by osmotic stress (Djibril et al. 2005; Farooq et al. 2008). The numerous physiological responses of plant to water deficits generally vary with the severity as well as the duration of water stress (Li, 2000; Correia et al., 2001; Pane and Goldstein, 2001; Pita and Pardes, 2001; Weigh, 2001; Edward and Wright, 2008).
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Water is imperative for plant growth and development. Water deficit stress, permanent or temporary, limits the growth and distribution of natural vegetation and the performance of cultivated plants more than any other environmental factor (Soriano et al. 2002, 2004; Farooq et al. 2008, Jaleel et al., 2009a,b). Although research and practices aimed at improving water stress resistance and water use efficiency have been carried out for many years, the mechanism involved is still not clear (Jaleel et al., 2008c,d). Further understanding and manipulating plant water relations and water stress tolerance can significantly improve plant productivity and environmental quality.
Water deficit stress can be defined as situation in which plant water potential and turgor are reduced enough to interface with normal functions (Zhu, 2002). Water stress is considered to be a moderate loss of water which leads to stomatal closure and limitation of gas exchange. Desiccation is a much more extensive loss of water which can potentially lead to gross disruption of metabolism and cell structure and eventually to the cessation of enzyme catalyzed reaction (Kage et al. 2004). Water stress is characterized by reduction of water content, turgor, total water potential, wilting,
closure of stomata, and decrease in cell enlargement and growth. Severe water stress may result in arrest of photosynthesis, disturbance of metabolism, and finally dying (Kamara et al. 2003).
Water stress influences plant growth at various levels of cell to community (Kiani et al. 2008). The quantity and quality of plant growth depend on cell division enlargement, and differentiation and all of these events are affected by water stress (Borsani et al. 2001; Kusaka et al. 2005; Jaleel et al., 2007c,d). It reduces plant growth inhibition of various physiological and biochemical processes, such as photosynthesis, respiration translocation, ion uptake, carbohydrates, nutrient metabolism and hormones (Lawlor et al. 2002).
Drought stress has adverse influence on water relations in Arachis hypogeae (Lawson et al. 2003), photosynthesis (Massacci, et al., 2008) and mineral nutrition, metabolism, growth and yield (Mohammadian, 2005). In addition, drought conditions influence the growth of weeds, agronomic management and nature and intensity of insects, pests and diseases (Monneveux et al., 2006; Nam et al., 2001).
Water use efficiency is traditionally defined as the ratio of dry matter accumulation to water consumption over a season. Increasing water use efficiency could theoretically affect plant growth. When water is limited, plants that use a finite water supply more efficient would positively affect plant productivity in peanut (Nayyar et al., 2006). Water use efficiency measurements may be made at three levels (i) in single leaf using gas exchange techniques (ii) in whole plants grown in containers and (iii) at the canopy level based on evapotranspiration in the field (Ogbonnaya et al., 2003).
Variation in water use efficiency amongest or within species can be assessed gravimetrically. However, reliable estimates of water use efficiency, under field conditions may be difficult, owing to the lack of technologies to asses the below ground biomass. But gravimetric technique can be adequately adopted to estimate in genotypic variation in pot culture experiments. Recent studies have shown that carbon isotope discrimination occurring during carbon assimilation by leaves is closely related to water use efficiency in various crops (Petropoulos et al. 2008), suggesting that carbon isotope discrimination technology can be used to screen genotypes for water use efficiency.
LEAF AREA DURATION
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Monthly variations in leaf area index have been observed for some of the trees species in accordance with seasonal variation in multipurpose agroforestry tree species (Colom and Vazzana, 2001; Thakur and Kaur, 2001). This variation was found to be the main cause of variation in productivity. Leaf area duration is reduced by decreasing water potential in Arachis hypogeae (Reddy et al., 2003).
Leaf area index is the ratio of the total leaf area of plant to the ground area covered by the plant. Leaf area index, which indicates the photosynthetic surface per unit area were found to increase rapidly 30 to 60 days after sowing and thereafter declined in both the years irrespective of cropping systems and nitrogen levels in maize (Shivay et al., 2002). The significant variation was reported in leaf area index in water stressed sunflower plants with respect to plantation time (Soriano et al., 2004). Leaf area decreased in podded plants and further reduced by water stress in Abelmoschus esculentum (Bhatt and Srinivasa Rao, 2005).
CUMULATIVE WATER TRANSPIRED
Cowpea cultivars were found to have high mean stomatal frequency on lower surface of leaf than upper surface which leads to reduced transpiration rate (Razmjoo et al. 2008). The decline in transpiration with the ageing of the leaf plays a very important role in regulating transpirational losses (Thakur and Kaur, 2001). Similar results were observed in barley (Samarah 2005), soybean (Samarah et al. 2006) and in Abelmoschus esculentus (Sankar et al,. 2008).
WATER USE EFFICIENCY
Water use efficiency can be increased by systems and limited water and land resources, inorder to increase the food production with least quantity of water supply (Debaeke and Aboudrare, 2004). Water use efficiency is a non linear function that increases under conditions of water stress in peanut (Ferreyra et al., 2003). Under salinity condition there was no cultivar differences in wheat to WUE, if the plants were given sufficient water in wheat (Shaheen and Hood-Nowotny, 2005).
Water use efficiency was found to be varying with genotypic differences in groundnut (Sankar et al. 2008). Gaspar et al. (2002) considered water use efficiency as an important trait for selection of drought
resistant varieties in plants. Water use efficiency was found to be varied in sunflower under water stress with respect to plantation time (Soriano et al., 2004).
Photosynthetic water use efficiency was found to be increased in potato during progressive soil drying in Solanum tuberosum (Liu et al., 2005). The existence of inter specific genetic differences in water use efficiency was reported in sympatric Populus species under water stress (Yin et al., 2005). When water is limited, plants that use a water supply more efficiently would grow more rapidly, in this case, high Water use efficiency would positively affect plant productivity in peanut (Wright et al., 1993). Similar results were observed in Phaseolus vulgaris (Martinez et al., 2007), Eucalyptus microtheca (Li et al., 2000), Populus cathayana (Tsialtas et al., 2001), Sympatric Oak species (Ponton et al., 2002) and Eucalyptus microtheca (Li and Wang, 2003).
Under water stress water use efficiency were found to be increased in different species. Similar results were reported in previous studies (Li et al., 2000; Amdt et al., 2001; Marron et al., 2002; Siemens and Zwiazek, 2003; Zhang et al., 2004). Increased water use efficiency was found in relation to osmotic adjustment in two populations of Atriplex halimus (Martinez et al., 2003).
NET ASSIMILATION RATE
Net assimilation is found to be unaffected under water stress in Asteriscus maritimus plants (Rodriguez et al., 2005). Net assimilation rate is estimation of canopy photosynthesis per unit leaf area and can be used as a measure of photosynthetic efficiency. Itââ‚¬â„¢s contribution of yield is not direct (Specht et al., 2001). The net assimilation rate was relatively lower under water stressed condition in Cluster bean (Vyas et al., 2001).
Net assimilation rate was found to be increased in maize plants when the cropping system and nitrogen levels were modified in the field (Amdt et al. 2001; Shivay et al., 2002). The decrease in photosynthetic rate strongly indicated stomatal closure as the factor for reduction during increased level of stress. The stomatal resistance significantly increased under water stress, whereas intercellular CO2 concentration significantly reduced under severe stress condition (Meenakshi Sundaravalli et al., 2005). Sharp et al. (2002) noted that the leaf enlargement, stomatal opening and photosynthesis are directly affected by the water stress treatment in Sorghum leaves. Sinaki et al. (2007) also observed that water stress leads to decline in net photosnthesis and altered chloroplast capacity. Water stress can also negatively affect the photosynthetic activity of the plants through inactivation of enzymes (Cabuslay et al. 2002; Chaves et al., 2002; Lawlor, 2002).
MEAN TRANSPIRATION RATE
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Rate of water loss was found to be decreased in Avocado cultivars under water deficit conditions due to increased some stomatal closure (Chartzoulakis et al., 2002). Similar case was found in Vaccinium myrtillus leaves (Chartzoulakis et al. 2002; Tahkokorpi et al., 2007). Transpiration efficiency with early plantings were consistently higher than those of late planting in sunflower under drought stress (Chaitanya et al. 2003; Soriano et al., 2004). Transpiration efficiency strongly influenced by the evaporative demand (Wu et al., 2008). Under rainfed condition with Cowpea genotyps belonging to different growth habit indicated that the determinate genotypes had higher values of transpiration rate as compared to indeterminate genotypes.
Harvest index found to be varied in drought stress sunflower with respect to plantation time (Soriano et al., 2004; Amarjit et al. 2005). In grain crops, avoiding the determinant effects of water deficits on the harvest index also minimizes the impact of the water limitation (Tahir et al. 2001). A two fold difference in harvest Index was imposed at different growth stages in water stressed sunflower (Soriano et al., 2002). A reduction in harvest Index was reported in sunflower during reproductive stages under water stressed conditions (Soriano et al., 2002). Long season genotype of sunflower has longer HI values than short season cultivars under rainfed conditions.
Greater plant fresh and dry weights under water limited conditions are desirable characters. A common adverse effect of water stress on crop plants is the reduction in fresh and dry biomass production. Plant productivity under drought stress is strongly related to the processes of dry matter partitioning and temporal biomass distribution (Kage et al., 2004). Diminished biomass due to water stress was observed in almost all genotypes of sunflower (Tahir & Mehid, 2001). However, some genotypes showed better stress tolerance than the
others. Mild water stress affected the shoot dry weight while shoot dry weight was greater than root dry weight loss under severe stress in sugar beet genotypes (Mohammadian et al., 2005). Reduced biomass was seen in water stressed soybean (Specht et al., 2001), Poncirus trifoliatae seedlings (Wu et al., 2008), common bean and green gram and Petroselinum crispum (Petropoulos et al., 2008). A moderate stress tolerance in terms of shoot dry mass plants was noticed in rice (Lafitte et al., 2007).
Harvest index varied among cultivars of groundnut under water stressed condition (Nautiyal et al., 2002; Bhatt and Srinivasa Rao, 2005; Reddy et al., 2004; Jaleel et al. 2008e, 2009c-e). Grain yield in finger millet showed significant positive association with harvest index and negative association with dates to maturity. Harvest index drastically reduced under water stress in spring wheat (Zhu, 2002; Pan et al., 2003).
Drought is a worldwide problem, constraining global crop production and quality seriously, and recent global climate change has made this situation more serious. Drought is also a complex physical-chemical process, in which many biological macromolecules and small molecules are involved, such as nucleic acids, proteins, carbohydrates, lipids, hormones, ions, free radicals, and mineral elements. In addition, drought is related to salt stress, cold stress, high-temperature stress, acid stress, alkaline stress, pathological reactions, senescence, growth, development, cell circle, UV-B damage, wounding, embryogenesis, flowering, and signal transduction. Although physiological mechanisms of drought tolerance are relatively well understood, further studies are essential to determine the physiological basis of plant water relations under water deficit stress. In addition to other factors, changes in water relation parameters are of paramount importance to drought mechanisms.