Abstract: A solution culture experiment combined with measurements of root elongation, adsorption-desorption, and streaming potential, were used to evaluate the toxicity of Cu²⁺ and Cd²⁺ to maize roots as related with the chemical forms of the metals adsorbed on root surfaces. The alleviative effects of coexisting cations and organic acids on Cu²⁺/Cd²⁺ toxicity to maize as well as the related mechanisms were also investigated. The results show that the toxicity of Cu²⁺ to maize roots was greater than that of Cd²⁺ at the same concentration. This was attributed to the fact that more Cu²⁺ was adsorbed on the root surface as exchangeable, complexed and precipitated forms compared with Cd²⁺. When the concentration of Cu²⁺ and Cd²⁺ was 1 μmol·L^-1, the relative root elongation rates of maize were reduced by 52.40% and 96.23%, respectively, compared with those of the control. When the metal concentration was 40 μmol·L^-1, the amounts of exchangeable, complexed and precipitated Cu²⁺ on the root surface were 1.22, 4.36 and 2.45 times that of Cd²⁺, respectively. At pH 4.5, coexisting cations of Ca²⁺, Mg²⁺, K⁺ or NH₄⁺ reduced the amount of heavy metals adsorbed on the root surface through competition for adsorption sites and thereby alleviating the toxicity of Cu²⁺ and Cd²⁺. Additionally, the alleviative effects of the divalent cations of Ca²⁺ and Mg²⁺ were greater than those of the monovalent cations of K⁺ and NH₄⁺. Furthermore, citric acid, oxalic acid, tartaric acid, and malic acid alleviated Cu²⁺ and Cd²⁺ toxicity to maize by forming stable complexes with the cations and reducing the amount of the metals adsorbed onto the roots. The alleviating effects of the organic acids on Cu²⁺ and Cd²⁺ toxicity followed the order: oxalic acid > citrate > tartaric acid ≈ malic acid, which was consistent with the complexation ability of these organic acids. Therefore, the toxicity of heavy metals to plant roots can be effectively alleviated by reducing the adsorption quantity of their exchangeable and complexed forms on root surfaces.

Key words: coexisting cation, organic acid, Cu²⁺/Cd²⁺, maize, adsorption-desorption, relative root elongation