Different ammonium-ion uptake,metabolism and detoxification efficiencies in two Lemnaceae

¹⁵N-Nuclear magnetic resonance spectroscopy was used to follow nitrogen assimilation and amino-acid production in Wolffia arrhiza (L.) Hork. ex. Wimmer, clone Golan exposed to 4.0 mM ¹⁵NH₄Cl solutions for 24 h. The main ¹⁵N-labelled metabolites were asparagine and glutamine, as well as substantial amounts of unreacted, intracellular NH₄⁺. These results were compared with those of a previous study on Lemna gibba L. clone Hurfeish (Monselise et al., 1987, New Phytol. 10, 341–345) with regard to NH₄⁺uptake, assimilation and detoxification efficiencies. Both species, grown under continuous white light, were capable of preferential uptake of NH₄⁺in the presence of nitrate. Relative growth rates indicate that both species tolerate increased levels of NH₄⁺, up to 10^–2 mol · 1^–1, with L. gibba showing a slightly greater tolerance. No ¹⁵N-labelled free NH₄⁺was detectable in L. gibba, while in W. arrhiza excess NH₄⁺was found within the cells. This fact indicates that L. gibba is more efficient in detoxification than W. arrhiza, presumably because of inability of W. arrhiza to regenerate the NH₄⁺traps, glutamate and aspartate, rapidly enough. This is also evident from the observation that addition of -ketoglutarate to the medium caused nearly complete assimilation of intracellular NH₄⁺in W. arrhiza. In both plants, addition of -ketoglutarate increased both NH₄⁺uptake and assimilation. Addition of l-methionine dl-sulfoximine, an inhibitor of glutamine synthetase inhibited NH₄⁺assimilation, while addition of azaserine, an inhibitor of glutamate synthase, resulted in ¹⁵N incorporation into the glutamine-amide position only. These results are consistent with the glutamine synthetase-glutamate synthase pathway being the major route of NH₄⁺assimilation in the two plants under the conditions used.Abbreviations AZA azaserine (O-diazoacetyl-l-serine) - GOGAT glutamine oxoglutarate amine transferase=]glutamate synthase E.C. 1.4.7. and E.C. 1.4.1.13. - GS glutamine synthetase E.C. 6.3.1.2. - -KG -ketoglutarate=2-oxoglutarate - MSO l-methionine dl-sulphoximine - NMR nuclear magnetic resonance - RGR relative growth rateThis article is dedicated to Professor Bernhard Schrader on the occasion of his 60th birthdayWe wish to thank Professor Robert Glaser for helpful discussions, and Mrs. Aliza Levkoviz and Mr. Gideon Raziel for skillful assistance.