The toxicity of HCrO and CrO to barley root elongation in solution culture: pH effect and modelling.

The influence of pH on the toxicity of Cr(VI) to barley root elongation was studied in solution culture to better understand the toxicity of different species of Cr(VI). Results showed that the values of EC50{CrO} (the free CrO that results in50% of barley root elongation with respect to the control) increased when the pH increased from 4.5 to 6.5; however, it was not significantly different in the high-pH range from 7.0 to 8.5. The nonlinear relationship between EC50{CrO} and OH activity indicated that OH competition with Cr(VI) on cell membrane ligands was not strong. There was a good linear relationship (R = 0.99) between the ratio of HCrO activity to CrO activity and Cr(VI) toxicity to barley root elongation when the toxicity of HCrO were considered, indicating that the observed toxicity of Cr(VI) in the high pH range may be caused by HCrO and CrO in solution. It was found that HCrO had a greater binding affinity than CrO on the biotic ligand sites. The logistic dose-response curves showed that consideration of Cr(VI) dose as HCrO and CrO significantly improved the data fit compared to consideration of the activity of HCrO or CrO only. The present study suggested that HCrO was highly toxic to the root of barely, and both HCrO and CrO species needed to be considered when predicting the toxicity of Cr(VI) under different pH conditions.