An in Vivo Nuclear Magnetic Resonance Investigation of Ion Transport in Maize (Zea mays) and Spartina anglica Roots during Exposure to High Salt Concentrations

Abstract

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 was investigated using 23Na and 31P nuclear magnetic resonance spectroscopy (NMR). Changes in the chemical shift of the pH-dependent 31P-NMR signals from the cytoplasmic and vacuolar orthophosphate pools were correlated with the uptake of sodium, and after allowing for a number of 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 maize root tips exposed to salt concentrations greater than 200 mM. The data were interpreted in terms of the ion transport processes that may be important during salt stress, and we concluded that the vacuolar alkalinization 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 treatment 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-induced activation of the tonoplast H+- ATPase. Sodium vanadate, an inhibitor of the plasmalemma H+- ATPase, stimulated the net uptake of sodium by maize root tips, and this was interpreted in terms of a reduction in active sodium efflux from the tissue. S. anglica root tips accumulated 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 appears that salt tolerance in Spartina is based in part on its ability to prevent the net influx of sodium chloride.