Nitrate and ammonium have different effects on many biochemical and physiological processes in plants, and at high concentrations this can lead to markedly different growth responses. Most plant species show reduced growth, smaller leaves and a stunted root system when exposed to high ammonium concentrations, and in severe cases this leads to chlorosis. Although well known, ammonium toxicity is poorly understood and is generally considered to be the result of one or more of the following effects; (i) ammonium‐induced mineral nutrient deficiency, arising from the impaired uptake of metal ions; (ii) secondary growth inhibition arising from the acidification of the rooting medium; (iii) alterations in intracellular pH and osmotic balance; (iv) uncoupling of photophosphorylation from electron transport, following the accumulation of ammonium in leaves; and (v) altered polyamine and phytohormone metabolism. These hypotheses are reviewed in the light of the available literature and experimental evidence from own experiments. It is concluded that no mechanism on its own provides an adequate explanation of the available data.

Analytical methods for probing plant metabolism are taking on new significance in the era of functional genomics and metabolic engineering. Among the available methods, nuclear magnetic resonance (NMR) spectroscopy is a technique that can provide insights into the integration and regulation of plant metabolism through a combination of in vivo and in vitro measurements. Thus NMR can be used to identify, quantify, and localize metabolites, to define the intracellular environment, and to explore pathways and their operation. We review these applications and their significance from a metabolic perspective. Topics of current interest include applications of NMR to metabolic flux analysis, metabolite profiling, and metabolite imaging. These and other areas are discussed in relation to NMR investigations of intermediary carbon and nitrogen metabolism. We conclude that metabolic NMR has a continuing role to play in the development of a quantitative understanding of plant metabolism and in the characterization of metabolic phenotypes.

In vivo 15N NMR spectroscopy was used to monitor the assimilation of ammonium by cell-suspension cultures of carrot (Daucus carota 1. cv Chantenay). The cell suspensions were supplied with oxygen in the form of either pure oxygen ("oxygenated cells") or air ("aerated cells"). In contrast to oxygenated cells, in which ammonium assimilation had no effect on cytoplasmic pH, ammonium assimilation by aerated cells caused a decrease in cytoplasmic pH of almost 0.2 pH unit. This led to a change in nitrogen metabolism resulting in the accumulation of 7-aminobutyric acid. The metabolic effect of the reduced oxygen supply under aerated conditions could be mimicked by artificially decreasing the cytoplasmic pH of oxygenated cells and was abolished by increasing the cytoplasmic pH of aerated cells. The activity of glutamate decarboxylase increased as the cytoplasmic pH declined and decreased as the pH recovered. These findings are consistent with a role for the decarboxylation of glutamate, a proton-consuming reaction, in the short-term regulation of cytoplasmic pH, and they demonstrate that cytoplasmic pH influences the pathways of intermediary nitrogen metabolism.

Stoichiometric modelling of leaf metabolism reveals metabolic and morphological determinants for introducing Crassulacean acid metabolism and alternative watersaving flux modes into a C3 leaf in different environments.

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