Phytotoxic effect of aluminium on maize root membranes

In this study, the effect of Al on the electrical membrane potential (Em) o f outer cortex root cells of three-day-old maize seedlings has been studied . The Em values of root cells ranged between -115 and -146 mV. Em of older cells towards the root base was considerably more negative than that of the young root tip cells. Experiments with NaCN + SHAM revealed that the diffu sion potential (E-D) of root tip cells is smaller than that of root base ce lls. In short term experiments Em has been rapidly and significantly depola rized by Al. The depolarization was concentration dependent and reached the maximum at 150 mu M Al. The extent of membrane depolarization by 100 mu M Al decreased continuously from the apex to the base of the root. Simultaneo usly, the number of cells without any response to Al considerably increased in the region from 10 to 35 mm behind the root tip. Interestingly, this ap parent insensitivity correlated well with the differentiation of the known maize root exodermis. No Em changes have been observed in cells more distan t than 40 mm from apex. The sensitivity of the cells to Al decreased in rad ial direction and when the tip of the microelectrode reached the interior o f the central cylinder Al induced no Em changes. Both the P-ATPase activato r fusicoccin (FC) and glucose diminished the depolarizing effect of Al on E m. Exposure of roots to Al treatment retarded K+ efflux from root tip segme nts and had no effect on K+ efflux from segments of the root base. The pres ent results suggest that, in short-term experiments, Al does not directly a ffect the electrogenic plasma membrane H+ ATPase.