Suppression of ammonium uptake by nitrogen supply and its relief during nitrogen limitation

Abstract

Barley (Hordeum vulgare, cv. Triumph), wheat (Triticum aestivum, cv. Kleiber) and oat (Avena sativa, cv. Tarok) were grown until day 20 in nitrate‐containing solutions or in nitrogen‐free solutions for periods up to 8 days immediately prior to day 20. They then were exposed for 4 h to complete, nitrate‐free solutions containing 0.5 or 2.0 mM ammonium (98 atom%15N). In all 3 species in 2 experiments, net ammonium uptake was low in plants grown continuously in nitrate, and increased 3 to 4‐fold with increasing nitrogen deprivation. Charge balance during net ammonium uptake was largely maintained by the sum of net potassium and net proton efflux. Variations in root ammonium concentration at the time of exposure to the ammonium solutions revealed no consistent pattern with net ammonium uptake, implying that a product of ammonium assimilation may serve as a negative effector for the uptake process. In nitrogen‐replete plants, and in those deprived of nitrogen for 2 days, the amounts of endogenous 14N‐ammonium recovered in the ambient 15N‐ammonium solution during the 4‐h uptake period were greater than the initial amounts of 14N‐ammonium present in the root tissue. Significant generation of 14N‐ammonium from endogenous organic nitrogen sources was thus evident in all 3 species.