Salt Stress-Induced Cytoplasmic Acidification and Vacuolar Alkalization in Nitellopsis obtusa Cells : In Vivo31P-Nuclear Magnetic Resonance Study

Time courses of cytoplasmic and vacuolar pH changes under salt stress were monitored by in vivo 31 P-nuclear magnetic resonance spectroscopy in intact cells of Nitellopsis obtusa . When cells were treated with 100 millimolar NaCl for 2 hours, the cytoplasmic pH deceased from 7.2 to 7.0, while the vacuolar pH increased from 4.9 to 5.2. This salt-induced breakdown of the pH gradient between the cytoplasm and the vacuole was also confirmed through direct measurements of change in vacuolar pH with a micro-pH electrode. We speculate that the intracellular pH changes induced by the salt stress mainly results from the inhibition of the H + -translocating pyrophosphatase in the vacuolar membrane, since this H + -translocating system is sensitive to salt-induced increase in the cytoplasmic [Na + ] and a simultaneous decrease in the cytoplasmic [K + ]. Since disturbance of the cytoplasmic pH value should have serious consequences on the homeostasis of living cells, we propose that the salt-induced intracellular pH changes are one of initial and important steps that lead to cell death.