Are root elongation assays suitable for establishing metallic anion ecotoxicity thresholds?

Metallic anions including antimonate (SbV), arsenate (AsV), chromate (CrVI), molybdate (MoVI), selenate (SeVI), tungstate (WV) and vanadate (VV) are important pollutants in the terrestrial environment due to their impacts on human and ecological health. It is essential that appropriate assays are used for derivation of toxicity models and guidance values, and to assess potential impacts on a site-specific basis. Root elongation is a simple and quick method for assessment of metal toxicity, yet there has been little to no validation. This study outlines results demonstrating low sensitivity of metallic anions in the often used 4-d root elongation test relative to 28-day nutrient culture assays. Therefore, root elongation assays may not be suitable for AsV, CrVI, MoVI, SbV, SeVI, and WV toxicity studies based on estimated toxicity parameters given the sensitivity of longer test assays. Only vanadate showed equivalent toxicity in the 4 d and 28 d assays. These results have significant implications for development of toxicity models and derivation of safe soil guidance values involving metallic anions.