Assessment of the Zn-Co mixtures rhizotoxicity under Ca deficiency: using two conventional mixture models based on the cell membrane surface potential.

Toxicity assessment of Zn-Co mixtures involves multiple ions interactions. The negative potential (ψ0) at the cell membrane surface (CMs) concentrated cationic toxicants (denoted {M(2+)}0) and influenced the rhizotoxicity of Co(2+) or Zn(2+). The single and joint rhizotoxicity of Co(2+) and Zn(2+) to wheat (Triticum aestivum L.) were examined, coupled with different Ca(2+) levels. Joint effects of Zn(2+), Co(2+) and Ca(2+) were estimated by the linearly extended concentration addition (CA) and response addition (RA) models. Incorporation of Ca(2+) in single metal toxicity assessment significantly enhanced the prediction accuracy (r(2) increased from 0.948 to 0.550 for Zn(2+) and from 0.903 to 0.611 for Co(2+), respectively). ψ0 affected the multiple metals toxicity in both conventional mixture models (r(2)=0.814 for CA model and 0.820 for RA model). Concretely, {Zn(2+)}0 alleviated the toxicity of {Co(2+)}0, while {Co(2+)}0 had non-significant effect on {Zn(2+)}0 toxicity. Growth responses to {Ca(2+)}0 were substantially affected by {Zn(2+)}0 and {Co(2+)}0. Ca addition in medium decreased the {M(2+)}0 by reducing the ψ0 negativity, moreover this addition alleviated Ca deficiency at CMs induced by Zn(2+) (or Co(2+)). These consistent results from both extended CA and RA models indicated that ψ0 provided a novel sight for understanding the rhizotoxicity of multiple metals.