Alleviation of rapid, futile ammonium cycling at the plasma membrane by potassium reveals K+-sensitive and -insensitive components of NH4+ transport

Futile plasma membrane cycling of ammonium (NH 4 + ) is characteristic of low-affinity NH 4 + transport, and has been proposed to be a critical factor in NH 4 + toxicity. Using unidirectional flux analysis with the positron-emitting tracer 13 N in intact seedlings of barley (Hordeum vulgare L.), it is shown that rapid, futile NH 4 + cycling is alleviated by elevated K + supply, and that low-affinity NH 4 + 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), NH 4 + influx (at an external [NH 4 + ] of 10 mM) of 92 μmol g −1 h −1 was observed, with an efflux:influx ratio of 0.75, indicative of rapid, futile NH 4 + cycling. Elevating K + supply into the low-affinity K + transport range (1.5-40 mM) reduced both influx and efflux of NH 4 + by as much as 75%, and substantially reduced the efflux:influx ratio. The reduction of NH 4 + fluxes was achieved rapidly upon exposure to elevated K + , within 1 min for influx and within 5 min for efflux. The channel inhibitor La 3+ decreased high-capacity NH 4 + influx only at low K + concentrations, suggesting that the K + -sensitive component of NH 4 + influx may be mediated by non-selective cation channels. Using respiratory measurements and current models of ion flux energetics, the energy cost of concomitant NH 4 + 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 NH 4 + flux mechanisms in plants.