Additive toxicity of zinc and arsenate on barley (Hordeum vulgare) root elongation.
Zinc (Zn) and arsenic (As) are typically present as mixed contaminants in mining-impacted areas; however, their joined effects have rarely been evaluated. The present study was set up to test whether the Zn2+ and H2 AsO4- (hereafter, As) mixture toxicity to plants is additive or whether interactions occur. Barley (Hordeum vulgare) root elongation was measured in resin buffered nutrient solutions. The design included ranges of single-element concentrations and combinations at 3 different Ca2+ concentrations (0.5 mM, 2.2 mM, and 15.0 mM) to vary the relative toxicity of Zn2+ . Increasing Ca concentrations decreased Zn toxicity, whereas As toxicity was unaffected by Ca. Root elongation was generally more affected in Zn-As mixtures than in corresponding single-element treatments. This is merely a joint additive effect, as 96% of the root elongation data were within a factor of 1.2 from predictions using the independent action (IA) or concentration addition (CA) model. The CA and IA predictions were similar, and data did not allow identification of equal or dissimilar modes of action. Small but significant Zn-As antagonisms were only found at high effects (>50% inhibition). The present study suggests that mixture effects of Zn and As are environmentally relevant and that current risk assessment underestimates toxicity in multielement-contaminated environments. The CA model can be used as a conservative model for risk assessment; however, for soil-grown plants, soil-exposed studies are needed. Environ Toxicol Chem 2017;36:1556-1562. © 2016 SETAC.