Blackwell Publishing Journal Backfiles 1879-2005
In N-starved (−N) fronds of Lemna gibba L. G 1, NH4+ uptake rates were several-fold those of NO3−-supplied (+N) fronds. NO3−, uptake in +N-plants was slow and not inhibited by addition of NH4+. However, in −N-plants with higher NO3− and still higher NH4+ uptake rates, addition of NH4+ immediately reduced the NO3− uptake rates to about one third until the NH4+ was consumed. The membrane potential (Em) decreased immediately upon addition of NH4+ in all fronds, but whereas depolarisation was moderate and transient in +N-plants, it was strong, up to 150 mV, in N-starved plants, where Em remained at the level of the K+ diffusion potential (ED) until NH4+ was removed. In N-starved plants NH4+ 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 NO3− uptake, but only in P- and N-starved plants, not after mere P starvation. Influx of NO3− and H2PO 4− into the negatively charged 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 NH4+ uniport, the linear component diffusion of NH4+ or NH3. Inhibition of anion/H+ cotransport by high NH4+ influx rates may be due to loss of the proton-driving force, Δμ̃H+, across the plasmalemma. Reversible inhibition by NH4+ of the H+ extrusion pump may contribute to the finding that Δμ̃H+ cannot be reconstituted in the presence of higher NH4+ concentrations.
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