INTERACTIONS OF CA2+ AND NACL STRESS ON THE ION RELATIONS AND INTRACELLULAR PH OF SORGHUM-BICOLOR ROOT-TIPS - AN IN-VIVO P-31-NMR STUDY

In vivo P-31-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 (Delta pH(tonoplast)) in Sorghum bicolor root tips exposed to 200 mM NaCl. The reduced Delta pH(tonoplast) in root tips exposed to 200 mM NaCl was primarily due to vacuolar alkalization rather than cytoplasmic acidification. Maintenance of the Delta pH(tonoplast) 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 P-i, vacuolar P-i, 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 CaSO, under NaCl stress were probably due to increased membrane permeability, as evidenced by the release of P to the incubation solution. The effect of supplemental Ca2+ on the Delta pH(tonoplast) 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(i)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.