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

Abstract

Toxicity assessment of Zn–Co mixtures involves multiple ions interactions. The negative potential (ψ0) at the cell membrane surface (CMs) concentrated cationic toxicants (denoted {M2+}0) and influenced the rhizotoxicity of Co2+ or Zn2+. The single and joint rhizotoxicity of Co2+ and Zn2+ to wheat (Triticum aestivum L.) were examined, coupled with different Ca2+ levels. Joint effects of Zn2+, Co2+ and Ca2+ were estimated by the linearly extended concentration addition (CA) and response addition (RA) models. Incorporation of Ca2+ in single metal toxicity assessment significantly enhanced the prediction accuracy (r2 increased from 0.948 to 0.550 for Zn2+ and from 0.903 to 0.611 for Co2+, respectively). ψ0 affected the multiple metals toxicity in both conventional mixture models (r2=0.814 for CA model and 0.820 for RA model). Concretely, {Zn2+}0 alleviated the toxicity of {Co2+}0, while {Co2+}0 had non-significant effect on {Zn2+}0 toxicity. Growth responses to {Ca2+}0 were substantially affected by {Zn2+}0 and {Co2+}0. Ca addition in medium decreased the {M2+}0 by reducing the ψ0 negativity, moreover this addition alleviated Ca deficiency at CMs induced by Zn2+ (or Co2+). These consistent results from both extended CA and RA models indicated that ψ0 provided a novel sight for understanding the rhizotoxicity of multiple metals.