Alleviation of rapid, futile ammonium cycling at the plasma membrane by potassium reveals K-sensitive and -insensitive components

Futile plasma membrane cycling of ammonium (NH4 ) is characteristic of low-affinity NH4 + transport, and has been proposed to be a critical factor in NH4 + toxicity. Using unidirectional flux analysis with the positronemitting tracer N in intact seedlings of barley (Hordeum vulgare L.), it is shown that rapid, futile NH4 + cycling is alleviated by elevated K supply, and that low-affinity NH4 + transport is mediated by a K-sensitive component, and by a second component that is independent of K. At low external [K] (0.1 mM), NH4 + influx (at an external [NH4 ] of 10 mM) of 92 mmol g h was observed, with an efflux:influx ratio of 0.75, indicative of rapid, futile NH4 + cycling. Elevating K supply into the low-affinity K transport range (1.5– 40 mM) reduced both influx and efflux of NH4 + by as much as 75%, and substantially reduced the efflux: influx ratio. The reduction of NH4 + fluxes was achieved rapidly upon exposure to elevated K, within 1 min for influx and within 5 min for efflux. The channel inhibitor La decreased high-capacity NH4 + influx only at low K concentrations, suggesting that the K-sensitive component of NH4 + influx may be mediated by non-selective cation channels. Using respiratory measurements and current models of ion flux energetics, the energy cost of concomitant NH4 + and K transport at the root plasma membrane, and its consequences for plant growth are discussed. The study presents the first demonstration of the parallel operation of K-sensitive and -insensitive NH4 + flux mechanisms in plants.