RHIZOTOXIC EFFECTS OF SILVER IN COWPEA SEEDLINGS

摘要

Silver (Ag) is highly toxic to aquatic organisms, including algae, invertebrate animals, and fish, but little information exists on Ag rhizotoxicity in higher plants. In two solution culture experiments with approximately 1,000 μM Ca(NO_3)_2 and 5 μM H_3BO_3 (pH 5.4), 20 to 80% of added Ag (≤2 μM) was lost from solution within approximately 30 min, with a further decrease after 48 h root growth. Using measured Ag concentrations at the start of the experiments, the median effective concentration (EC50) for root elongation rate of cowpea (Vigna unguiculata [L.] Walp. cv. Caloona) was 0.0I0μM Ag in the first 4h of exposure (0.021 μM in the first 8h). This demonstrates that Ag (as Ag~+) is rapidly rhizotoxic to cowpea seedlings at concentrations similar to those that are toxic to freshwater biota. Rupturing of rhizodermal and outer cortical layers was evident after 48 h with 0.13 to 0.57 μM Ag initially in solution, being most severe at 0.13 or 0.25 μM Ag. An additional experiment showed that ruptures were first evident after 20 h exposure to 0.17 μM Ag, with increased severity of rupturing over time. The rhizotoxic effects of Ag are similar to those of some other trace metals (e.g., Cu, Al, La) that bind strongly to hard ligands and weakly to soft ligands. The similarity of rupturing effects, despite the difference in strong binding to soft ligands by Ag and to hard ligands by the other metals, suggests a distinctive metabolic effect of Ag that binds only weakly to hard ligands.