PLANT PHYSIOLOGY , Vol 102, Issue 2 629-638, Copyright © 1993 by American Society of Plant Biologists
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ENVIRONMENTAL AND STRESS PHYSIOLOGY |
An in Vivo Nuclear Magnetic Resonance Investigation of Ion Transport in Maize (Zea mays) and Spartina anglica Roots during Exposure to High Salt Concentrations
C. M. Spickett, N. Smirnoff and R. G. Ratcliffe
Department of Plant Sciences, University of Oxford, South Parks Road, Oxford, OX1 3RB, United Kingdom (C.M.S., R.G.R.)
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.
