Influx and Efflux of Phosphorus in Roots of Wheat Plants in Non-Growth-Limiting Concentrations of Phosphorus

Abstract

The regulation of net phosphorus uptake was studied in wheat plants at ambient non-growth-limiting P-concentrations. Wheat (Triticum aestivum cv. Klein Atalaya) seedlings were grown from germination in culture solutions containing 0.05, 0.5 and 5.0 mol m−3 phosphate. Only small increments in plant P-concentration and specific accumulation rate for phosphorus were found when ambient P-concentration was increased 100 times. P-influx, estimated by ³²P-uptake, was markedly greater with increased external P-concentration, but only small changes in V_max and no changes in K_m were found. Indirect estimation of P-efflux in a time-course of ³²P-uptake, and direct P-efflux measurements in ‘wash out’ experiments indicated that P-efflux markedly increased in higher ambient P-concentration. The increase in P-efflux almost completely neutralized the higher P-influx observed in 5.0 mol m⁻³ relative to 0.05 mol m⁻³ phosphate. It is postulated that in non-limiting P-concentration net P-uptake is mainly controlled by P-efflux.