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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, (14)N-nuclear magnetic resonance spectra were recorded with and without (1)H-decoupling and information on the subcellular distribution of NH 4 (+) was obtained from a lineshape analysis of the (1)H-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.

Original publication




Journal article



Publication Date





359 - 367