Different ammonium-ion uptake, metabolism and detoxification efficiencies in two Lemnaceae - A ¹⁵N-nuclear magnetic resonance study

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 α-ketoglutarate to the medium caused nearly complete assimilation of intracellular NH₄⁺in W. arrhiza. In both plants, addition of α-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.