Protective Effect of External Ca²⁺ 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.