Journal Article

Intracellular pH regulation in maize root tips exposed to ammonium at high external pH

J. Gerendás and R.G. Ratcliffe

in Journal of Experimental Botany

Published on behalf of Society for Experimental Biology

Volume 51, issue 343, pages 207-219
Published in print February 2000 | ISSN: 0022-0957
Published online February 2000 | e-ISSN: 1460-2431 | DOI:
Intracellular pH regulation in maize root tips exposed to ammonium at high external pH

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Ammonium‐induced changes in the cytoplasmic and vacuolar pH values of excised maize (Zea mays L.) root tips, measured by in vivo 31P nuclear magnetic resonance (NMR) spectroscopy, were correlated with the ammonium content of the tissue, determined by 14N NMR. Calculations based on these measurements indicated that the pH changes observed during exposure to 10 mM ammonium for 1 h at pH 9.0, and in the recovery following the removal of the external ammonium supply, were largely determined by the influx and efflux of the weak base NH3. Carboxylate synthesis, detected by both in vivo 13C NMR and the incorporation of [14C]bicarbonate, was stimulated by the ammonium‐induced alkalinization of the root tips, but the contribution that this proton‐generating process made to pH regulation during and after the ammonium treatment was quantitatively insignificant. Similarly, ammonium assimilation, which was shown to occur via the proton‐generating glutamine synthetase/glutamate synthase pathway using in vivo 15N NMR, was also quantitatively insignificant in comparison with the large changes in ammonium content that occurred during the ammonium treatment and subsequent recovery. The results are discussed in relation to several recent studies in which ammonium was used to perturb intracellular pH values, and it is argued (i) that a new method for probing the subcellular compartmentation of amino acids, based on an ammonium‐induced alkalinization of the cytoplasm may be difficult to implement in dense heterogeneous tissues; and (ii) that observations on the apparently proton‐consuming effect of ammonium assimilation in rice root hairs may actually reflect unusually rapid assimilation.

Keywords: Ammonium efflux; cytoplasmic pH; GS/GOGAT pathway; NMR spectroscopy; vacuolar pH.

Journal Article.  8446 words.  Illustrated.

Subjects: Plant Sciences and Forestry

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