Salinity stress: physiological constraints and adaptive mechanisms.

A significant part of the world's land area is salt-affected, including areas in which crops and pastures are grown for food and forage. Growth and yield of most crops is reduced by salinity, and only halophytes are able to handle large amounts of salt without penalty. This chapter summarizes our current knowledge of physiological mechanisms conferring plant adaptive responses to salinity. The classification of saline soils is given with causes of primary and secondary types of salinity. Major physiological constraints are then summarized, and physiological and genetic diversity of plant responses to salinity are presented. Key physiological and anatomical mechanisms conferring salinity tolerance in plants are then analysed in detail, with emphasis on how salt uptake, transport and accumulation in tissues within the plant are controlled. This chapter shows that plants have evolved numerous mechanisms to prevent accumulation of toxic Na+ concentrations in leaves, and to regulate concentrations of Na+, K+ and Cl- within the various cell compartments. This ability is complemented by mechanisms enabling efficient osmotic adjustment and maintenance of cell turgor, as well as mechanisms of coping with oxidative stress imposed by salinity. A more complete physiological and genetic understanding of these processes will enable targeted breeding for new salt-tolerant plants for the future.