Mechanisms of Aluminum Tolerance in Wheat : An Investigation of Genotypic Differences in Rhizosphere pH, K, and H Transport, and Root-Cell Membrane Potentials.

Control of rhizosphere pH and exclusion of Al by the plasma membrane have been hypothesized as possible mechanisms for Al tolerance. To test primarily the rhizosphere pH hypothesis, wheat cultivars (Triticum aestivum L. ;Atlas 66' and ;Scout'), which differ in Al tolerance, were grown in either complete nutrient solution, or 0.6 millimolar CaSO(4), with and without Al at pH 4.50. A microelectrode system was used to simultaneously measure rhizosphere pH, K(+), and H(+) fluxes, and membrane potentials (E(m)) along the root at various distances from the root apex. In complete nutrient solution, the rhizosphere pH associated with mature root cells (measured 10-40 millimeters from the root apex) of Al-tolerant ;Atlas 66' was slightly higher than that of the bulk solution, whereas roots of Al-sensitive ;Scout' caused a very small decrease in the rhizosphere pH. In CaSO(4) solution, no significant differences in rhizosphere pH were found between wheat cultivars, while differential Al tolerance was still observed, indicating that the rhizosphere pH associated with mature root tissue is not directly involved in the mechanism(s) of differential Al tolerance. In Al-tolerant ;Atlas 66', growth in a CaSO(4) solution with 5 micromolar Al (pH 4.50) had little effect on net K(+) influx, H(+) efflux, and root-cell membrane potential measured in cells of mature root tissue (from 10-40 mm back from apex). However, in Al-sensitive ;Scout', Al treatment caused a dramatic inhibition of K(+) influx and both a moderate reduction of H(+) efflux and depolarization of the membrane potential. These results demonstrate that increased Al tolerance in wheat is associated with the increased ability of the tolerant plant to maintain normal ion fluxes and membrane potentials across the plasmalemma of root cells in the presence of Al.