A, 1984, Ammonium uptake in Lemna gibba G 1, related membrane potential changes, and inhibition of anion uptake, -Physiol, Plant, 61;[369][370][371][372][373][374][375][376] In N-starved (-N) fronds oi Lemna gibba L, G 1, NHJ uptake rates were several-fold those of NOi-supplied ( + N) fronds, NOj uptake in +N-plant,s was slow and not inhibited by addition of NHJ, However, in-N-plants with higher NO^ and still higher NH| uptake rates, addition of NH| immediately reduced the NO5 uptake rates to about one third until the NHJ was consumed. The membrane potential (E,,,) decreased immediately upon addition of NH4+ in all fronds, but whereas depolarisation was moderate and transient in -t-N-plants, it was strong, up to 150 mV, in N-starved plants, where E,,, remained at the level of the K' diffusion potential (Ep) until NHJ was removed. In N-starved plants NHJ uptake and membrane depolarisation showed the same concentration dependence, except for an apparent linear component for uptake. Phosphate uptake was inhibited by NH4+ similarly to NO5 uptake, but only in P-and N-starved plants, not after mere P starvation. Influx of NO5 and H:PO4 into the negatively eharged cells of Lemna is mediated by anion/H* cotransport, but NH4+ influx can follow the electrochemical gradient. Its saturating component may reflect a carrier-mediated NHJ uniport, the linear component diffusion of NH4+ or NH,, Inhibition of anion/H+ eotransport by high NH4+ influx rates may be due to loss of the proton-driving force, AflH% across the plasmalemma. Reversible inhibition by NHJ of the H+ extrusion pump may contribute to the finding that A|:iH+ cannot be reconstituted in the presence of higher NHJ concentrations.Additional key words -H+ extrusion pump, nitrate uptake, phosphate uptake, W. R. Ullrich (reprint requests) and S. Lesch, Insl. fiir Boianik, Teclmische Hochschule, Sclmittspahnstras.se 3, FRG; M, Botaniska In.st., Stocklwlms Univ., Sweden; A. Novacky,
