Protective effect of external Ca⁺ on elongation and the intracellular concentration of K⁺ in intact mung bean roots under high NaCl stress

The effects of Ca²⁺ in the external medium on intact mung bean roots under high NaCl stress were investigated. With increasing external concentrations of NaCl, mung bean roots showed suppression of elongation and a decrease in the intracellular concentration of K⁺. Addition of Ca²⁺ to the external medium alleviated the inhibition of root elongation under the high NaCl stress and maintained a high intracellular concentration of K⁺ in the elongating region of the roots. This counter effect of Ca²⁺ against the NaCl stress on roots was correlated with the ratio of [Ca²⁺]/[Na⁺]² in the external medium. A value above 5.0 × 10^-4 mM^-1 resulted in almost complete recovery of root elongation under various high concentrations of NaCl. Root elongation for 24 h under NaCl stress was correlated with the extent to which the intracellular concentration of K⁺ was in excess of 10 mM. Maintenance of an adequate concentration of K⁺ in root cells is essential for root elongation under salt stress. These findings indicate that Ca²⁺ prevents the leakage of intracellular K⁺ and thereby supports the elongation of roots under salt stress.