The role of calcium in salt toxicity

Salt toxicity comprises osmotic and ionic components both of which can severely affect root and shoot growth. Uptake of Na⁺ across the plasma membrane is very fast resulting in physiological effects on extracellular as well as intracellular sites. Sodium reduces binding of Ca²⁺ to the plasma membrane, inhibits influx while increasing efflux of Ca²⁺, and depletes the internal stores of Ca²⁺ from endomembranes. These changes in the cell Ca²⁺ homeostasis are suggested here to be the primary responses to salt stress that are perceived by root cells. Salt would almost instantly reduce the amount of Ca²⁺ being transferred to the leaf cells, with Ca²⁺ activity dropping and Na⁺ activity rising in the apoplasm of leaf cells. This Ca²⁺ signal would be transported to leaves together with, if not preceding, the signal of limited water supply. Hormonal signals are likely to be secondary in nature and caused by the Na⁺-related disturbance of the root cell Ca²⁺ homeostasis. Ameliorative effects of supplemental Ca²⁺ on salt stress are exerted through preventing Na⁺-related changes in the cell Ca²⁺ homeostasis.