Changes in the cytoplasmic and vacuolar pH in intact cells of mung bean root-tips under high-NaCl stress at different external concentrations of Ca⁺ ions

The cytoplasmic pH and the vacuolar pH in root-tip cells of intact mung bean seedlings under high-NaCl stress were measured by in vivo ³¹P-nuclear magnetic resonance (³¹P-NMR) spectroscopy. When roots were incubated with high levels (100 mM) of NaCl at the control external concentration (0.5 mM) of Ca²⁺ ions, the vacuolar pH increased rapidly from 5.6 to 6.2 within 3 h, while the cytoplasmic pH only decreased by a mere 0.1 pH unit even after a 24-h incubation under high-NaCl conditions. The increase in vacuolar pH induced by the high-NaCl stress was diminished by an increase in the external concentration of Ca²⁺ ions from 0.5 mM to 5 mM. The intracellular concentration of Na⁺ ions in the root-tip cells increased dramatically upon perfusion of the root cells with 100 mM NaCl, and high external levels of Ca²⁺ ions also suppressed the in flow of Na⁺ ions into the cells. The vacuolar alkalization observed in salt-stressed roots may be related to the inhibition of an H⁺-translocating pyrophosphatase in the tonoplast, caused by the increase in the cytoplasmic concentration of Na⁺ ions. It is suggested that, although the vacuolar pH increased markedly under salt stress, the cytoplasmic pH was tightly regulated by some unidentified mechanisms, such as stimulation of the H⁺-translocating ATPase of the plasmalemma, in roots of mung bean under salt stress.