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 Ca 2+ ions ([Ca2+]ex). The extrusion of protons was gradually enhanced in the roots exposed to 100 mM NaCl, and high [Ca2+]ex diminished this e nhancement of the extrusion. Vesicles of plasmalemma and tonoplast wer e prepared from the roots and the H+-translocating ATPase (H+-ATPase) activities associated with the two types of membrane and the H+-pyroph osphatase (H+-PPase) activity of the tonoplast were assayed. The plasm alemma ATPase was stimulated in parallel with dramatic increases in th e intracellular concentration of Na+([Na+]in). High [Ca2+]ex prevented the increase in [Na+]in and diminished the stimulation of ATPase acti vity. The tonoplast ATPase showed a rapid response to salt stress and was similarly stimulated even at high [Ca2+]ex. The activities of both ATPases were, however, insensitive to concentrations of Na+ ions up t o 100 mM. By contrast, H+-PPase activity of the tonoplast was severely inhibited with increasing [Na+]in under salt stress and recovered wit h high [Ca2+]ex. These findings suggest that high-NaCl stress increase s the intracellular concentration of Na+ ions in mung bean roots, whic h inhibits the tonoplast H+-PPase, and the activity of the plasmalemma H+-ATPase is thereby stimulated and regulates the cytoplasmic pH.