Modeling the interaction and toxicity of Cu-Cd mixture to wheat roots affected by humic acids, in terms of cell membrane surface characteristics.

Responses of wheat (Triticum aestivum L.) seedling roots to the mixtures of copper (Cu), cadmium (Cd) and humic acids (HA) were investigated using the solution culture experiments, focusing on the interaction patterns between multiple metals and their influences on root proton release. A concentration-addition multiplication (CA) model was introduced into the modeling analysis. In comparison with metal ion activities in bulk-phase solutions, the incorporation of ion activities at the root cell membrane surfaces (CMs) (denoted as {Cu2+ }0 and {Cd2+ }0 ) into the CA model could significantly improve their correlation with RRE (relative root elongation) from 0.819 to 0.927. Modeling analysis indicated that the co-existence of {Cu2+ }0 significantly enhanced the rhizotoxicity of {Cd2+ }0 , while no significant effect of {Cd2+ }0 on the {Cu2+ }0 rhizotoxicity. 10 mg/L HA stimulated the root elongation even under metal stress. Although high concentration of metal ions inhibited the root proton release rate (ΔH+ ), both the low concentration of metal ions and HA treatments increased the values of ΔH+ . In HA-Cu-Cd mixtures, actions of metal ions on ΔH+ values were varied intricately among treatments but well modeled by the CA model. We concluded from the CA models that the electrostatic effect is vitally important for explaining the effect of {Cu2+ }0 on the rhizotoxicity of {Cd2+ }0 , while it plays no unique role in understanding the influence of {Cd2+ }0 on the rhizotoxicity of {Cu2+ }0. Thus our study provide a novel way for modeling multiple metals behaviors in the environment and understanding the mechanisms of ion interactions.