Negative charges at cell membrane surfaces (CMS) create a surface electrical potential (psi(0)) that affects ion concentrations at the CMS and consequently affects the phytotoxicity of metallic cations and metalloid anions in different ways. The zeta potentials of root protoplasts of wheat (Triticum aestivum), as affected by the ionic environment of the solution, were measured and compared with the values of psi(0) calculated with a Gouy-Chapman-Stern model. The mechanisms for the effects of cations (H(+), Ca(2+), Mg(2+), Na(+), and K(+)) on the acute toxicity of Cu(2+) and As(V) to wheat were studied in terms of psi(0). The order of effectiveness of the ions in reducing the negativity of psi(0) was H(+) > Ca(2+) approximately Mg(2+) > Na(+) approximately K(+). The calculated values of psi(0) were proportional to the measured zeta potentials (r(2) = 0.93). Increasing Ca(2+) or Mg(2+) activities in bulk-phase media resulted in decreased CMS activities of Cu(2+) ({Cu(2+)}(0)) and increased CMS activities of As(V) ({As(V)}(0)). The 48-h EA50{Cu(2+)}(b) ({Cu(2+)} in bulk-phase media accounting for 50% inhibition of root elongation over 48 h) increased initially and then declined, whereas the 48-h EA50{As(V)}(b) decreased linearly. However, the intrinsic toxicity of Cu(2+) (toxicity expressed in terms of {Cu(2+)}(0)) appeared to be enhanced as psi(0) became less negative and the intrinsic toxicity of As(V) appeared to be reduced. The psi(0) effects, rather than site-specific competitions among ions at the CMS (invoked by the biotic ligand model), may play the dominant role in the phytotoxicities of Cu(2+) and As(V) to wheat.