Intracellular pH and proton-transport in barley root cells under salt stress: In vivo ³¹P-NMR study

Salt stress-induced changes of intracellular pH and in levels of phosphorous compounds were monitored in intact root tips of barley seedlings (Hordeum vulgare cv. Akashinriki) by in vivo ³¹P-nuclear magnetic resonance (NMR) spectroscopy. Vacuolar alkalization was observed after treatment with both 300 and 500 mM NaCl. Much of the observed apparent alkalization of the cytoplasm was eliminated when the effect of Na⁺ ions on the titration curve was considered. Within 1 h after the initiation of salt stress, levels of glucose-6-phosphate and UDP-glucose decreased markedly, and such decreases might lead directly or indirectly to cell death. Simultaneous measurements of the external and intracellular pH revealed the promotion of external acidification and internal alkalization during salt stress. Possible mechanisms of Na⁺/H⁺ antiport at the tonoplast and the role of proton-pump in the plasma membrane are discussed.