ABSTRACT
Abiotic stress is a highly probable occurrence for almost any plant growing under either natural or cultivated conditions. Some of the most important abiotic stresses that limit world crop production are drought, heat, salinity, freezing, and acidic soil (Blum, 1988). Mild stresses occur frequently, sometimes daily in the case of drought, and result in reduced growth, productivity, and quality of crops. Severe stresses can lead to catastrophic losses. Dudal (1976) estimated that only 10 percent of the world’s arable land may be categorized as free of stress. About 20 percent of the land is under some kind of mineral stress, 26 percent is affected by drought stress, and 15 percent by freezing stress. Sanchez and Salinas (1981) estimated that approximately 55 percent of the soils in tropical America, 39 percent in tropical Africa, and 37 percent in tropical Asia are acidic, representing 1.6 billion hectares. A significant portion of the terrestrial environment is affected by high levels of salt. Including the major deserts, there are approximately 4 million km2 of salt-affected land in the world (Flowers et al., 1977). Saline and sodic soils cover about 10 percent of the total arable land in over 100 countries of the world and, of the cultivated lands, about 23 percent are saline and 37 percent are sodic (Tanji, 1990). Loss of land to salt accumulation through irrigated agriculture has been estimated to be at least several hundred km2 a year in India alone (Flowers et al., 1977). In California, 20 percent of irrigated agricultural land is affected to some extent by salt (Bernstein, 1975).
