Observations on the subcellular distribution of the ammonium ion in maize root tissue using in-vivo 14N-nuclear magnetic resonance spectroscopy

We show that the pH dependence of the base-catalysed exchange rate of the ammonium ion provides a basis for discriminating between the cytoplasmic and vacuolar pools of ammonium in plant tissues. In vivo, ¹⁴N-nuclear magnetic resonance spectra were recorded with and without ¹H-decoupling and information on the subcellular distribution of NH ₄ ⁺ was obtained from a lineshape analysis of the ¹H-coupled spectrum. We applied this method to maize (Zea mays L.) root tissues and found that: (i), the cytoplasmic ammonium concentration was low, which was in accord with the large activity of glutamine synthetase present in the roots; and (ii), inhibition of glutamine synthetase with methionine sulphoximine increased the cytoplasmic ammonium concentration, and led to the appearance of ammonium in the xylem sap.Abbreviations GS glutamine synthetase - MSO l-methionine sulphoximine - NMR nuclear magnetic resonance - Pi inorganic phosphate On secondment to the Department of Plant Sciences, University of Oxford.We acknowledge the financial support of the Agricultural and Food Research Council. R.B. Lee also thanks the Department of Plant Sciences, University of Oxford, for hospitality.