Ammonium uptake in <i>Lemna gibba</i> G 1, related membrane potential changes, and inhibition of anion uptake

In N‐starved (−N) fronds of Lemna gibba L. G 1, NH 4 + uptake rates were several‐fold those of NO 3 − ‐supplied (+N) fronds. NO 3 − , uptake in +N‐plants was slow and not inhibited by addition of NH 4 + . However, in −N‐plants with higher NO 3 − and still higher NH 4 + uptake rates, addition of NH 4 + immediately reduced the NO 3 − uptake rates to about one third until the NH 4 + was consumed. The membrane potential (E m ) decreased immediately upon addition of NH 4 + 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 E m remained at the level of the K + diffusion potential (E D ) until NH 4 + was removed. In N‐starved plants NH 4 + uptake and membrane depolarisation showed the same concentration dependence, except for an apparent linear component for uptake. Phosphate uptake was inhibited by NH 4 + similarly to NO 3 − uptake, but only in P‐ and N‐starved plants, not after mere P starvation. Influx of NO 3 − and H 2 PO 4 − into the negatively charged cells of Lemna is mediated by anion/H + cotransport, but NH 4 + influx can follow the electrochemical gradient. Its saturating component may reflect a carrier‐mediated NH 4 + uniport, the linear component diffusion of NH 4 + or NH 3 . Inhibition of anion/H + cotransport by high NH 4 + influx rates may be due to loss of the proton‐driving force, Δμ̃H + , across the plasmalemma. Reversible inhibition by NH 4 + of the H + extrusion pump may contribute to the finding that Δμ̃H + cannot be reconstituted in the presence of higher NH 4 + concentrations.