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

In vivo P-31-NMR measurements showed that supplemental Ca2+ (5.0 mM Ca SO4) decreased the magnitude of the NaCl-induced reduction of the pH g radient across the tonoplast (Delta pH(tonoplast)) in Sorghum bicolor root tips exposed to 200 mM NaCl. The reduced Delta pH(tonoplast) in r oot tips exposed to 200 mM NaCl was primarily due to vacuolar alkaliza tion rather than cytoplasmic acidification. Maintenance of the Delta p H(tonoplast) may be important for salinity tolerance since the trans-t onoplast H+ electrochemical gradient is the putative driving force for Na+ transport from the cytoplasm into the vacuole via a Na+/H+ antipo rt. Supplemental Ca2+ also reduced net Na accumulation, and maintained the levels of K, cytoplasmic P-i, vacuolar P-i, glucose-6-P, and tota l P in root tips exposed to 200 mM NaCl. The decreases in these solute s in root tips supplied with 0.5 mM CaSO, under NaCl stress were proba bly due to increased membrane permeability, as evidenced by the releas e 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, a nd/or (ii) reduce Na+ inhibition of the tonoplast H+-PP(i)ase, as sugg ested by some investigators. These findings are consistent with the hy pothesis that supplemental Ca2+ helps maintain membrane function in ro ot cells of NaCl-stressed non-halophytes.