Developing Crops with Tolerance to Salinity and Drought Stress

Availability of water is the most important factor for crop productivity. A vast area (more than 50 million hectares) of agricultural land throughout the world suffers from recurring droughts, resulting in poor crop productivity (Carter, 1975). An equally large area of land is affected by high salinity. Even though irrigated agriculture has increased significantly during the past twenty years, the high capital cost of this process and the resulting increase in salinity is making this approach difficult to adopt. Furthermore, excessive irrigation is lowering the water tables, reducing water availability even more. Drought and salinity are formidable obstacles to the development of new varieties that can give sufficient yield under water stress conditions (Boyer, 1982). Some plants have evolved adaptations to water deficit and high salinity. These adaptations encompass a wide variety of plant characteristics (McCue and Hanson, 1990), including developmental and structural traits, time of flowering, rooting patterns, leaf waxiness, and physiological mechanisms such as the ability to exclude salt or the compartmentalization of ions within the cell (Binzel et al., 1988). Obviously, these are Multigenic traits, and most of them are determined by gene products that have not yet been characterized. The Multigenic nature of the phenotypes has thwarted attempts to characterize these mechanisms at the genetic level and has hindered efforts to produce osmotolerant plants by traditional breeding and somaclonal variations (Vasil, 1990). Among the biochemical traits in the adaptation of plants to water stresses, synthesis and accumulation of compatible osmolytes and changes in patterns of carbon and nitrogen metabolism are most important. Plants accumulate energy-rich metabolites under water stress; the most prevalent of these are proline and betaines (Yancey et al., 1982). Concentration of K+ and organic solutes (primarily polyols) has been shown to increase in direct proportion to changes in osmotic stress in many bacteria, algae, and higher plants. With the recent advances in genetic transformation of crop plants, genes encoding entire biosynthetic pathways or that augment the rate-limiting step in an adaptive process can now be transferred to any crop plant.