15N NMR SPECTROSCOPIC STUDY OF AMMONIUM ION ASSIMILATION BY SPIRODELA OLIGORRHIZA—LEMNACEAE—AS AFFECTED BY LIGHT AND CARBON SUPPLY IN GREEN AND EXTOLIATED PLANTS

ABSTRACT Nitrogen assimilation and amino acid production in Spirodela oligorrhiza plants exposed to 30 mM 15NH4Cl was studied using l5N NMR spectroscopy. Green and etiolated plants were studied under different light regimes and in the presence of added carbon, either as sucrose or as α-ketoglutarate. Etiolated plants are capable of ammonium assimilation and, as in green plants, this occurs via the glutamine synthetase/glutamine oxoglutarate amine transferase (GS/GOGAT) and the aspartate aminotransferase/asparagine synthetase pathways. The major assimilation products in both etiolated and green plants were glutamine and asparagine. Thus our results confirm that N-amides are key detoxification products when plants are exposed to external ammonium ion, and act as storage reservoirs or sinks for assimilated ammonium. In plants grown under continuous light, ammonium ion was taken up and assimilated to completion. L-methionine DL-sulfoximine, a GS inhibitor, inhibited ammonium ion assimilation but not its uptak...

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