Phytotoxicity of Cu and Cd to the roots of four different wheat cultivars as related to charge properties and chemical forms of the metals on whole plant roots.

The relationship between the chemical forms of Cu and Cd adsorbed on the roots of different wheat cultivars and their phytotoxic effects on the plants were investigated. The wheat varieties Dunmaiwang (DMW), Tekang 6 (TK6), Zhongmai895 (ZM895), and Chaojixiaomai (AK68) were used. The zeta potentials of wheat roots, measured by the streaming potential method, were used to characterize root charge properties. Results indicated that the changes in zeta potential at pH 4.01-6.61 were 14.7, 15.53, 13.01, and 12.06 mV for ZM895, AK68, DMW, and TK6, respectively. The negative charge and functional groups on ZM895 and AK68 roots were greater than on DMW and TK6 roots, which led to more exchangeable and complexed Cu and Cd on ZM895 and AK68 roots and increased Cu and Cd toxicity compared to DMW and TK6. Coexisting cations, such as Ca, Mg, K, and NH, alleviated Cu and Cd toxicity to wheat roots through competition for adsorption sites on the roots, which decreased exchangeable and complexed Cu and Cd on wheat roots. The Ca and Mg were most effective in alleviating heavy metal toxicity and they decreased exchangeable Cu on AK68 roots by 39.14% and 47.82%, and exchangeable Cd by 8.51% and 28.23%, respectively.