Ion Homeostasis in NaCl Stress Environments

Homeostasis can be defined as the tendency of a cell or an organism to maintain internal steady state, even in response to any environmental perturbation or stimulus tending to disturb normality, because of the coordinate responses of its constituent components. Typically, ions constantly flux in and out of cells in a controlled fashion with net flux adjusted to accommodate cellular requirements, thus creating an ionic homeostasis. When plant cells are exposed to salinity, mediated by high NaCl concentrations, kinetic steady states of ion transport for Na+ and Cland other ions, such as K+ and Ca2+, are disturbed (Binzel et al., 1988). High apoplastic levels of Na+ and Clalter aqueous and ionic thermodynamic equilibria, resulting in hyperosmotic stress, ionic imbalance, and toxicity. Thus, it is vital for the plant to re-establish cellular ion homeostasis for metabolic functioning and growth, that is, to adapt to the saline environment. Comparisons of what have been interpreted to be adaptive responses among various species lead to the conclusion that some salt-tolerant plants have evolved specialized complex mechanisms that allow adaptation to saline stress conditions. In fact, these unique mechanisms, such as salt glands, exist in few plant species and cannot be presumed to be ubiquitously functional for salt adaptation of all plants. However, intrinsically cellular-based mechanisms appear to be common to all genotypes and are a requisite for salt tolerance. Of paramount importance are those mechanisms that function to regulate ion homeostasis while mediating osmotic adjustment through the accumulation and intracellular compartmentation of ions that are predominant in the external environment. In this update we will focus principally on Na+ homeostasis in sodic environments; however, we also include discussions of H+, K+, Ca2+, and Clbecause of the interrelationship of these ions with Na+ homeostasis. Ion transport processes across the plasma membrane and the tonoplast will be emphasized because these are presumed to be most essential for the control of intracellular Na+ uptake and vacuolar compartmentation.