Cortical Cell Fluxes of Phosphate in Excised Root Segments of Allium cepa L.

Compartmental analysis was used to study content and transmembranefluxes of phosphate in excised root segments in a nutrient solutioncontaining 1.0 mol phosphate m^–3, at pH 5.6. The resultingradioactivity content versus time curve deviated from the criteriadetermining a relationship conforming to first order kinetics,since the final rate constant was an order of magnitude lowerthan that exhibited by the curve for efflux versus time. Thedata were manipulated so as to bring about conformation to theappropriate criteria, revealing a large, slowly exchanging poolthat is considered to represent assimilated phosphate or storedpolyphosphate. Subsequent calculations, using the modified data,suggested that H₂PO₄ was absorbed by a phosphate pump at theplasmalemma, but was actively extruded from the vacuole afterpassive entry. The consequences of pH effects in the cell wall,cytoplasm and vacuole in determining the concentration of H₂PO^–₄in each phase are discussed. Key words: Phosphate compartmentation, pH, efflux