Enantioselective phytotoxicity of metolachlor against maize and rice roots.

Rac-metolachlor, a widely used chloracetanilide herbicide, is now being replaced by S-metolachlor in many countries. The enantioselective effects of rac- and S-metolachlor on root growth of maize and rice was studied in hydroponics. Visible morphological changes in root growth were observed after treatment with rac- or S-metolachlor. The main root and lateral roots were shorter in length, and the number of lateral roots was reduced. The half inhibition (IC(50,5d)) values for root length of rac- and S-metolachlor were 18.86 and 10.61 μM, respectively, for maize, and 7.33 and 5.35 μM, respectively, for rice. The root system activity after treatment with rac- or S-metolachlor was lower than that of the control, while the root membrane permeability was higher. The activities of superoxide dismutase, peroxidase, and catalase in the roots were lower after rac- or S-metolachlor treatment compared to those of the control, while the malondialdehyde content was higher. After rice was treated with 3.1 μM rac- or S-metolachlor, the cell wall separated from the cell membrane, and some destruction of nuclei and organelles was observed. The entire cell was destroyed after treatment with 12.4 μM rac- or S-metolachlor. The results showed that S-metolachlor has stronger effects than rac-metolachlor on crop roots.