THE ROLE OF LOW-MOLECULAR-WEIGHT ORGANIC-ACIDS IN THE MECHANISM OF INCREASED ZINC TOLERANCE IN SILENE VULGARIS (MOENCH) GARCKE

The possible role of low molecular weight organic acids in the mechanism of increased zinc tolerance is investigated in Silene vulgaris (Moench) Garcke. In the leaves, the malate concentration is higher in zinc-tolerant plants than in zinc-sensitive ones, but no consistent differences are observed in the roots. Therefore, since the mechanism of increased zinc tolerance operates also in the roots themselves, a primary role for malate in the mechanism of increased zinc tolerance is doubtful. Exposure to zinc does not affect the malate concentration in the plants. The citrate concentration in both the roots and leaves is higher in tolerant plants than in sensitive ones. A clear-cut effect of zinc on the concentration of citrate in the plants is not found. Citrate does not seem to play an important role in the sequestration of zinc in root cells, in view of its low concentration in the roots of tolerant plants. The possible role of citrate as a carrier for zinc in the cytosol is discussed. Oxalate, the most abundant organic acid, might play an important role in the sequestration of zinc, because of its high concentration in the plants. However, this cannot explain increased zinc tolerance, because the oxalate concentration is higher in the sensitive plants than in the tolerant ones. Neither succinate nor maleate play a prominent role in the mechanism of increased zinc tolerance. Malate and citrate are the only organic acids detected in the xylem fluid. The malate concentration in the xylem fluid is the same in sensitive and tolerant plants and it is not affected by the concentration of external zinc. The citrate concentration in the xylem fluid is about three times higher in the tolerant control plants than in the sensitive ones, but it decreases to the same level as in the sensitive plants after exposure to zinc. Differences in the citrate concentrations in the xylem fluid of sensitive and tolerant plants are not related to differences in the allocation of zinc.