Mixture toxicity of copper, cadmium, and zinc to barley seedlings is not explained by antioxidant and oxidative stress biomarkers

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 5d or 14d 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. (c) 2016 SETAC