The effect of glutamine and asparagine on net NH4 + uptake in young wheat plants

Wheat plants grown during 10 days in the absence of N were pretreated with 1.0 eq m^-3 of methionine, asparagine or glutamine and/or 1.0 eq m^-3 MSX⁴ or 0.17 eq m^-3 DON. Net NH₄⁺ uptake was measured both in the presence or in the absence of the amino acid or enzyme inhibitor used in the pretreatment. The effect of met, asn and gln on net K⁺ uptake was also studied using K⁺-depleted plants. Changes in the contents of root free NH₄⁺, asn, gln and the activities of GS, PEP-carboxylase, NAD⁺-GDH and NADH-GDH were determined. Net NH₄⁺ uptake in gln and asn pretreated plants was markedly, and sometimes completely suppressed provided uptake was measured in the presence of the amides. On the other hand, the met pretreated plants absorbed only 35% less NH₄⁺ than the control. When NH₄⁺ uptake was measured in the absence of the amino acids, only those plants pretreated with asn showed a marked suppression of net uptake during the first 120 min. None of the 3 amino acids tested significantly inhibited K⁺ uptake. Free NH₄⁺ concentration in roots of N-starved plants increased after 4 h incubation with gln, asn or MSX in the absence of external NH₄⁺. Nevertheless, no correlation was observed between root NH₄⁺ concentration and the extent of net NH₄⁺ uptake suppression. The inhibitory effect exerted by asn decreased when it was supplied together with MSX or DON. Pretreatments with gln or asn in the absence of external NH₄⁺ significantly increased the level of asn in the roots, while that of gln remained unchanged. It is concluded that asn and gln specifically suppress net NH₄⁺ uptake in wheat, although it is not clear wether they act only from the root exterior, or through an endogenous pool exhibiting fast turn-over.Abbreviations AUR ammonium uptake rate - DON 6-diazo-5-oxo-L-norleucine - GDH glutamic dehydrogenase - GOGAT oxoglutarate- glutamine aminotransferase - GS glutamine synthetase - MSX L-methionine sulfoximine - PEP phosphoenolpyruvate - PVPP polyvinylpolypyrrolidone