Cortical Cell Fluxes of Phosphate in Excised Root Segments ofAllium cepaL.

Abstract

Ron, M. M., Macklon, A. E. S. and Sim, A. 1988. Cortical cell fluxes of phosphate in excised root segments of Allium cepa L.—J. exp. Bot. 39: 1699-1707. Compartmental analysis was used to study content and transmembrane fluxes of phosphate in excised root segments in a nutrient solution containing 10 mol phosphate m 3, at pH 5-6. The resulting radioactivity content versus time curve deviated from the criteria determining a relationship conforming to first order kinetics, since the final rate constant was an order of magnitude lower than that exhibited by the curve for efflux versus time. The data were manipulated so as to bring about conformation to the appropriate criteria, revealing a large, slowly exchanging pool that is considered to represent assimilated phosphate or stored polyphosphate. Subsequent calculations, using the modified data, suggested that HjPO^ was absorbed by a phosphate pump at the plasmalemma, but was actively extruded from the vacuole after passive entry. The consequences of pH effects in the ceil wall, cytoplasm and vacuole in determining the concentration of H2PC>4 in each phase are discussed.