Identification of hydroxyl copper toxicity to barley (Hordeum vulgare) root elongation in solution culture.

The effect of pH on the acute toxicity of Cu to barley (Hordeum vulgare) root elongation was investigated in solution culture. The results showed that the median effective concentrations (EC50s; i.e., the concentration that reduced root elongation by 50% based on free Cu(2+) activity) were not significantly different in the low-pH range from 4.5 to 6.5, but in the high-pH range from 7.0 to 8.0, a significant effect of pH on EC50s was found. The nonlinear relationship between EC50 and H(+) activity in the present study indicated that the increased toxicity with increasing pH in solution may not be caused by decreasing H(+) competition. When we take account of CuOH(+) activities, a good linear relationship (r(2) > 0.97) between the ratio of CuOH(+) activity to free Cu(2+) activity and acute Cu toxicity to barley root elongation was achieved, which indicated that the observed toxicity in the high-pH range may be caused by CuOH(+) plus free Cu(2+) in solution. Linear-regression analysis suggested CuOH(+) had a greater binding affinity than Cu(2+) at the biotic ligand sites. The logistic dose-response curve showed that expressing the Cu dose as Cu(2+) + 2.92.CuOH(+) improved the data fit significantly compared to consideration of the free Cu(2+) activity only. Thus, our results suggest CuOH(+) was highly toxic to barley root elongation. The enhanced toxicity of CuOH(+) therefore needs to be considered when modeling the effect of pH on Cu toxicity to barley for exposures having pH greater than 6.5.