Interactions of Ca²⁺and NaCI stress on the ion relations and intracellular pH ofSorghum bicolorroot tips: Anin vivo³¹P-NMR study

In vivo31P-NMR measurements showed that supplemental Ca2+ (5.0 mM CaSO4) decreased the magnitude of the NaCl-induced reduction of the pH gradient across the tonoplast (δpHtonoplast) in Sorghum bicolor root tips exposed to 200 mM NaCl. The reduced (δpHtonoplast) in root tips exposed to 200 mM NaCI was primarily due to vacuolar alkalization rather than cytoplasmic acidification. Maintenance of the (δpHtonoplast) may be Important for salinity tolerance since the trans-tonoplast H+ electrochemical gradient is the putative driving force for Na+ transport from the cytoplasm into the vacuole via a Na+/H+ antiport. Supplemental Ca2+ also reduced net Na accumulation, and maintained the levels of K, cytoplasmic P1 vacuolar P1 glucose-6-P, and total P in root tips exposed to 200 mM NaCl. The decreases in these solutes in root tips supplied with 0.5 mM CaSO4 under NaCl stress were probably due to increased membrane permeability, as evidenced by the release of P to the incubation solution. The eftect of supplemental Ca2+ on the (δpHtonoplast) may be related to the reduced net influx of Na+, which in turn could (i) lower tonoplast Na+/H+ antiport activity, and/or (ii) reduce Na+ inhibition of the tonoplast H+PP, ase, as suggested by some investigators. These findings are consistent with the hypothesis that supplemental Ca2+ helps maintain membrane function in root cells of NaCl-stressed non halophytes.