ABSTRACT We present the first characterization of K(+) optimization of N uptake and metabolism in an NH(4)(+)-tolerant species, tropical lowland rice (cv. IR-72). (13)N radiotracing showed that increased K(+) supply reduces futile NH(4)(+) cycling at the plasma membrane, diminishing the excessive rates of both unidirectional influx and efflux. Pharmacological testing showed that low-affinity NH(4)(+) influx may be mediated by both K(+) and non-selective cation channels. Suppression of NH(4)(+) influx by K(+) occurred within minutes of increasing K(+) supply. Increased K(+) reduced free [NH(4)(+)] in roots and shoots by 50-75%. Plant biomass was maximized on 10 mm NH(4)(+) and 5 mm K(+), with growth 160% higher than 10 mm NO(3)(-)-grown plants, and 220% higher than plants grown at 10 mm NH(4)(+) and 0.1 mm K(+). Unlike in NH(4)(+)-sensitive barley, growth optimization was not attributed to a reduced energy cost of futile NH(4)(+) cycling at the plasma membrane. Activities of the key enzymes glutamine synthetase and phosphoenolpyruvate carboxylase (PEPC) were strongly stimulated by elevated K(+), mirroring plant growth and protein content. Improved plant performance through optimization of K(+) and NH(4)(+) is likely to be of substantial agronomic significance in the world's foremost crop species.