Metal contamination is mostly a mixture of different metals, and these multicomponent mixtures can produce significant mixture effects. The present study was set up to investigate the toxicity of multiple metal mixtures of Cu, Ni, Cd, and Zn to plants at metal doses individually causing low-level phytotoxic effects. Barley (Hordeum vulgare L.) root elongation toxicity tests were performed in resin-buffered nutrient solutions to control metal speciation. Treatments included single-metal concentrations and binary, ternary, and quaternary mixtures. Mixtures of different metals at free ion concentrations, each causing <10% inhibition of root elongation, yielded significant mixture effects, with inhibition reaching up to 50%. The independent action (IA) model predicted mixture toxicity statistically better than the concentration addition (CA) model, but some synergisms relative to the IA model were observed. These synergisms relative to IA were most pronounced in quaternary mixtures and when the dose-response curves had steep slopes. Generally, antagonistic interactions relative to the CA model were observed. Increasing solution Zn concentrations shifted metal interactions (CA based) from additive or slightly synergistic at background Zn concentrations to antagonistic at higher Zn concentrations, suggesting a protective effect of Zn. Overall, the present study shows that the CA model can be used as a conservative model to predict metal mixture toxicity to barley. Environ Toxicol Chem 2016;35:2483-2492. © 2016 SETAC.
The analysis of metal mixture toxicity to plants is complicated by mutual interactions. In the present study, mixture effects of zinc (Zn), cadmium (Cd), and copper (Cu) on barley (Hordeum vulgare L.) root elongation were analyzed using oxidative stress parameters. The hypothesis was that toxic mixture effects on plant growth are better explained by biochemical parameters than by exposure information, because the former excludes interactions among metals for root uptake. Barley seedlings were exposed for 5 d or 14 d to these metals in nutrient solutions, added in isolation and as mixtures. Root elongation in Cu+Cd mixtures was well predicted from free metal ion concentrations in solution, using concentration addition (CA) or independent action (IA) reference models. In contrast, Zn acted antagonistically when combined with Cu and/or Cd, relative to both CA and IA. This protective effect of Zn correlated with the biomarkers measured in the long-term experiment; oxidative stress (indicated by malondialdehyde level, for example) decreased after addition of Zn. In addition, it was found that some biomarkers were sensitive to both Cu and Cd dosed in isolation, but not to Cu+Cd mixtures. Overall, the exposure explained mixture effects better than most of the 16 measured biomarkers (i.e., the biochemical effects). It is concluded that these biomarkers are not robust indicators for metal mixture toxicity, potentially because different metals have different parallel modes of action on growth that are insufficiently indexed by the biomarkers. Environ Toxicol Chem 2017;36:220-230. © 2016 SETAC.
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