Phosphate fluxes, compartmentation and vacuolar speciation in root cortex cells of intactAgrostiscapillaris seedlings: effect of non-toxic levels of aluminium

Abstract

From compartmental analysis of 32 P elution measurements, concentrations and fluxes of orthophosphate were estimated for root cortical cells of intact seedlings of the indigenous grass Agrostis capillaris L. cv. Highland when in complete nutrient solution containing 10 mmol m -3 or 100 mmol m -3 phosphate with or without 3.7 and 37 mmol m -3 Al, respectively, during loading and elution. When plotted as counts min -1 remaining in the tissue as a function of time, the data failed to meet the criteria for first order kinetics. Transformation of the data to meet the kinetic criteria gave corrected values for compartmental concentrations and fluxes of phosphate, and estimates of the otherwise unresolved slowly exchanging compartment within the vacuole considered to be the cause of the discrepancy in flux analysis. In the control, the discrepancy was considered due to sequestration of phosphate with Ca in the vacuole and a small, but not significant, increase in sequestered P occurred in the presence of Al, the presence of which was confirmed by X-ray microanalysis. A chemical speciation model was used to demonstrate, for various values of pH and carboxylic acid concentration, the possibility that phosphate was precipitated in root cell vacuoles as Ca and Al phosphates (hydroxy-apatite and variscite). The ecological significance of the ability of A. capillaris plants to make efficient use of scarce P resources by minimizing the sequestration of P by Al in vacuoles, compared with Lolium perenne, was recognized.