Copper pollution may occur in acidic soils where the low pH leads to release of Al and Mn in soil solution, which could interact with Cu toxicity. Very little information exists regarding the influence of toxic cations on the phytotoxicity of Cu. In this study, we tested the hypothesis that phytotoxicity of Cu2+ may be overestimated in acidic soils due to synergism between Al or Mn and Cu toxicity. Rhizotoxicity of Al, Mn, and Cu to wheat seedlings was studied in well-defined nutrient solutions, with these elements present singly or in combination. Toxicity was expressed on a solution metal-activity basis, with metal activities calculated using GEOCHEM-PC and verified using Donnan dialysis. Of the three ions, Cu2+ was the most rhizotoxic, with activities of Cu2+, Al3+, and Mn2+ resulting in a 25% reduction in root elongation of 0.12, 1.26, and 211 μM, respectively. Although there was no interaction between Mn2+ and Cu2+ toxicity, Cu2+ was significantly less toxic on a relative basis in the presence of Al3+. Thus, critical thresholds for soil solution Cu2+ activity determined in acidic soils will be underprotective compared to soils that contain low concentrations of soluble Al (e.g., limed or nonacidic soils).
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