AN INVIVO NUCLEAR-MAGNETIC-RESONANCE INVESTIGATION OF ION-TRANSPORT IN MAIZE (ZEA-MAYS) AND SPARTINA-ANGLICA ROOTS DURING EXPOSURE TO HIGH-SALT CONCENTRATIONS

The response of maize (Zea mays L.) and Spartina anglica root tips to exposure to sodium chloride concentrations in the range 0 to 500 mm wa s investigated using Na-23 and P-31 nuclear magnetic resonance spectro scopy (NMR). Changes in the chemical shift of the pH-dependent P-31-NM R signals from the cytoplasmic and vacuolar orthophosphate pools were correlated with the uptake of sodium and after allowing for a number o f complicating factors we concluded that these chemical shift changes indicated the occurrence of a small cytoplasmic alkalinization (0.1-0. 2 pH units) and a larger vacuolar alkalinization (0.6 pH units) in mai ze root tips exposed to salt concentrations greater than 200 mm. The d ata were interpreted in terms of the ion transport processes that may be important during salt stress, and we concluded that the vacuolar al kalinization provided evidence for the operation of a tonoplast Na+/H-antiport with an activity that exceeded the activity of the tonoplast H+ pumps. The intracellular pH values stabilized during prolonged tre atment with high salt concentrations, and this observation was linked to the recent demonstration (Y. Nakamura, K. Kasamo, N. Shimosato, M. Sakata, E. Ohta [1992] Plant Cell Physiol 33: 139-149) of the salt-ind uced activation of the tonoplast H+-ATPase. Sodium vanadate, an inhibi tor of the plasmalemma H+-ATPase, stimulated the net uptake of sodium by maize root tips, and this was interpreted in terms of a reduction i n active sodium efflux from the tissue. S. anglica root tips accumulat ed sodium more slowly than did maize, with no change in cytoplasmic pH and a relatively small change (0.3 pH units) in vacuolar pH, and it a ppears that salt tolerance in Spartina is based in part on its ability to prevent the net influx of sodium chloride.