Stimulation of the extrusion of protons and H⁺-ATPase activities with the decline in pyrophosphatase activity of the tonoplast in intact mung bean roots under high-NaCl stress and its relation to external levels of Ca²⁺ ions

Extrusion of protons as a response to high-NaCl stress in intact mung bean roots was investigated at different external concentrations of Ca²⁺ ions ([Ca²⁺]_ex). The extrusion of protons was gradually enhanced in the roots exposed to 100 mM NaCl, and high [Ca²⁺]_ex diminished this enhancement of the extrusion. Vesicles of plasmalemma and tonoplast were prepared from the roots and the H⁺-translocating ATPase (H⁺-ATPase) activities associated with the two types of membrane and the H⁺-pyrophosphatase (H⁺-PPase) activity of the tonoplast were assayed. The plasmalemma ATPase was stimulated in parallel with dramatic increases in the intracellular concentration of Na⁺([Na⁺]_in). High [Ca²⁺]ex prevented the increase in [Na⁺]in and diminished the stimulation of ATPase activity. The tonoplast ATPase showed a rapid response to salt stress and was similarly stimulated even at high [Ca²⁺]M. The activities of both ATPases were, however, insensitive to concentrations of Na⁺ ions up to 100 HIM. By contrast, H⁺-PPase activity of the tonoplast was severely inhibited with increasing [Na⁺]in under salt stress and recovered with high [Ca²⁺]ex. These findings suggest that high-NaCl stress increases the intracellular concentration of Na⁺ ions in mung bean roots, which inhibits the tonoplast H⁺-PPase, and the activity of the plasmalemma H⁺-ATPase is thereby stimulated and regulates the cytoplasmic pH.