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

The regulation of net phosphorus uptake was studied in wheatplants at ambient non-growth-limiting P-concentrations. Wheat(Triticum aestivum cv. Klein Atalaya) seedlings were grown fromgermination in culture solutions containing 0.05, 0.5 and 5.0mol m–3 phosphate. Only small increments in plant P-concentrationand specific accumulation rate for phosphorus were found whenambient P-concentration was increased 100 times. P-influx, estimatedby ³²P-uptake, was markedly greater with increased externalP-concentration, but only small changes in V_max and no changesin K_m were found. Indirect estimation of P-efflux in a time-courseof ³²P-uptake, and direct P-efflux measurements in ‘washout’ experiments indicated that P-efflux markedly increasedin higher ambient P-concentration. The increase in P-effluxalmost completely neutralized the higher P-influx observed in5.0 mol m^–3 relative to 0.05 mol m^–3 phosphate.It is postulated that in non-limiting P-concentration net P-uptakeis mainly controlled by P-efflux. Key words: Net P uptake, ³²P, kinetic parameters