ALUMINUM EFFECTS ON TRANSMEMBRANE POTENTIAL IN CELLS OF FIBROUS ROOTS OF SUGAR-BEET

The effect of aluminium (Al) on the electrical transmembrane potential of epidermal and outer cortical root cells of intact seedlings of sugar beet (Beta vulgaris L. cv. Monohill) was studied. The potential difference to the surrounding medium was recorded with microelectrodes inserted into the vacuoles (PD(v)) and cytoplasm (PD(c)) of intact roots. Both long-term effects of AlCl3 (100-mu-M present during cultivation) and immediate effects of AlCl3 (10, 50, or 100-mu-M present in the assay medium), were measured. The effect of Al was measured at pH 4.0, 5.0 and 6.5 in order to obtain information on the toxicity of different Al forms existing at different pH values. Low pH and/or the presence of AlCl3 during cultivation caused large depolarizations of the PD(v). Since the immediate effect of 2,4-dinitrophenol (DNP) on the resting potential of cells from Al-cultivated plants was negligible, it is likely that Al affects the metabolic component of the transmembrane potential. Aluminium also had an immediate effect on the PD in root cells of plants cultivated without Al. Addition of 10 or 50-mu-M Al to the assay medium caused hyperpolarization of PD(v) in the presence of 0.5 mM Ca2+ at all pH values studied, depolarization of PD(c) at pH 6.5, and hyperpolarization of PD(c) at lower pH. At 1 mM Ca2+, or in the presence of K+ (greater-than-or-equal-to 2 mM), however, the same Al concentrations had little effect on PD(c). The strongest depolarizing effects of 10 or 50-mu-M Al in short-term treatments were obtained at pH 6.5, and were probably due to the soluble species Al(OH)3, which is more frequent at pH 6.5 than at a lower pH. Addition of 50-mu-M Al caused alkalinization of the root medium at pH 6.5, but not at pH 4.0. Therefore, it is possible that Al at pH 6.5 is bound to, or translocated across, the membrane without the accompanying hydroxide ions. It is likely that most of the Al is bound to the root cells, since removal of Al from the buffer surrounding the roots did not cause the changed PD values to return to the original values. Aluminium also interacts with effects of Ca2+ and K+ on the membrane potential, since changes in PD, induced by changes in concentrations of Ca2+ and K+ are different in the absence and presence of Al.