Effect of pH on Orthophosphate Uptake by Corn Roots.

Orthophosphate (Pi) influx in washed corn roots was studied with experimental conditions allowing a distinction of pH effects on Pi ionization in the medium and on the transport system itself. There appeared to be no relationship between the pH dependencies of membrane potential, H(+) secretion, and (32)Pi influx. The Pi uptake versus pH curves were compared to the calculated ones describing the concentrations of the different ionized Pi forms in the medium and in the cell walls; the latter were obtained using the theoretical model described by Sentenac and Grignon (1981) Plant Physiol 68: 415-419). The conclusion was that the transported form is H(2)PO(4) (-) and the concentration sensed by the transport system is the local one. The ionic compositions of experimental media were manipulated to ensure constant pH and various H(2)PO(4) (-) concentrations, or constant H(2)PO(4) (-) concentration and various pH values in the walls. The kinetic analysis of the results in the micromolar range showed that the transport system has an intrinsic sensitivity to pH, and is switched from a low activity state at pH > 6 to a high activity one at pH < 4 (pH in the walls). This change could be triggered by the protonation of a group with pK 5.5.