Modeling of selenite toxicity to wheat root elongation using biotic ligand model: Considering the effects of pH and phosphate anion.

It has not been well understood that the binding affinity and potential toxicity of different chemical forms of selenite (Se(IV)), which are predominant forms of selenium with plant availability. The influences of pH and major anions on Se(IV) toxicity to wheat root elongation were determined in solutions and modeled based on the biotic ligand model (BLM) and free ion activity model (FIAM) concepts. Results showed that EC50[Se(IV)]T values increased from 164 to 273 μM as the pH raised from 4.5 to 8.0, indicating the increase of pH induced weakened Se(IV) toxicity. The EC50{SeO32-} values increased from 0.019 to 71.3 μM while the EC50{H2SeO3} values sharply decreased from 2.08 μM to 0.760 nM with the pH increasing from 4.5 to 8.0. The effect of pH on Se(IV) toxicity could be explained by the changes of Se(IV) species in different pH solutions as H2SeO3, HSeO3- and SeO32- were differently toxic to wheat root elongation. The toxicity of Se(IV) decreased with increasing H2PO4- activity but not for SO42-, NO3- and Cl- activities, indicating that only H2PO4- had a competitive effect with Se(IV) on the binding sites. A site-specific BLM was developed to count in effects of pH and H2PO4-, and stability constants of H2SeO3, HSeO3-, SeO32- and H2PO4- to the binding sites were obtained: log [Formula: see text]  = 4.96, log [Formula: see text]  = 3.47, log [Formula: see text]  = 2.56 and log [Formula: see text]  = 2.00. Results implied that BLM performed much better than FIAM in the wheat root elongation prediction when coupling toxic species H2SeO3, HSeO3-, SeO32-, and the competitions of H2PO4- for the binding sites while developing the Se(IV)-BLM.