Observations on the subcellular distribution of the ammonium ion in maize root tissue using in-vivo (14)N-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, (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.