Sorption parameters as a predictor of arsenic phytotoxicity in Australian soils

- Sorption binding constants are the best predictor of arsenic phytotoxicity.
- Toxicity end-points for arsenic are correlated but quantitatively different.
- Root elongation does not show sensitivity to arsenic exposure.

Arsenic (As) is a mobile and ecotoxic metalloid that is of serious concern to the environment. In this study, As phytotoxicity was studied using a dose-response approach for seven contrasting soils considering 3 end-points (shoot biomass, root elongation and chlorophyll content) and focusing on predictors of toxicity. Root elongation study was carried out for 4 days using both Cucumis sativus L. (cucumber) and Triticum aestivum L. (wheat) and shoot end-points with a 4 week a pot study using cucumber only. Root elongation of cucumber was a substantially less sensitive indicator to As than data from the 4 weeks pot study. Effective concentrations (50%)(EC50) from cucumber root elongation studies were overall 1.6 times higher than the 4 week shoot data. Cucumber was however considerably more sensitive to wheat. Given the large discrepancy in phytotoxicity end points for 7 soils, root elongation data for ecotoxicological assessment should be treated with some caution. Arsenic phytotoxicity was strongly related to the sorption constants of each of the seven soils in our study. Both root elongation and shoot data were related strongly to Freundlich partitioning constants (Kf) (L/kg). Wheat and cucumber root elongation had R2 values 0.90 and 0.91 respectively, while cucumber shoot data was 0.79. The Kf values were related to soil pH and also EC50 data and, thus, shows that As phytotoxicity in our study was primarily controlled by sorption reactions. The rate of As bioaccumulation to cucumber shoots depended heavily on the soil under consideration. Chlorophyll and carotenoid content of cucumber shoots increased with As content in 3 soils and decreased in other soils.