Drought, salinity, and extreme temperatures are major environmental stresses that limit crop productivity and quality. Drought is a problem in 45 percent of the world’s geographical area, where 38 percent of the world’s human population lives (Bot et al., 2000). In rainfed agricultural ecosystems, crops experience either continuous drought stress or intermittent cycles of water availability and stress. Fresh water is increasingly becoming a scarce resource, and plants account for about 65 percent of global fresh water use (Postel et al., 1996). Soil salinity limits crop production in about 20 percent of irrigated land (Flowers and Yeo, 1995). The problem of salinity is aggravated in dry land as salts accumulate in the root zone due to high evaporative demand and insufficient leaching of ions due to inadequate rainfall. Temperature extremes (temperatures lower or higher than the physiological optimum of a crop plant) exacerbate drought and salinity stresses and are also a limiting factor of production even in irrigated environments. Temperate crops need freezing tolerance for their survival and production. Tropical and subtropical crops such as rice, maize, soybean, cotton, and tomato are susceptible to chilling injury when ambient temperatures fall below a nonfreezing critical threshold (0 to 15°C). High-temperature stress is one of the major limiting factors of crop production in the subtropic and tropics. Heat stress affects grain quality and yield in 40 percent of the irrigated wheat-growing areas of the world (Fischer and Byerlee, 1991).
Among these abiotic stresses, drought is highly unpredictable in terms of occurrence, severity, duration, and its timing in relation to crop phenology.