Research On The Ecotoxicology And Enantioselectivity Of Metolachlor On Maize And Rice

More than 40% of currently used pesticides are chiral compounds, and the percentage of pesticides that are chiral compounds is increasing with the introduction of more complex structures. Despite the great public concerns associated with the use of chiral insecticides, the environmental significance of currently used chiral insecticides is poorly understood. The worldwide research on the enantioselective behavior of chiral contaminants in the environment is still at its early stage, more attentions should be given to environmental behavior of enantiomers.Metolachlor is an amide herbicide with a high activity and selectivity. Metolachlor was first introduced into the market as a racemic product, which contains two R-enantiomers and two S-enantiomers. Herbicidal activity is mainly from the S-enantiomer, and R-enantiomer is mutagenic. S-metolachlor is now widely used instead of rac-metaolachlor for the former is more effective than the latter in herbicidal activity. Toxicology of maize (Yedan13, Zea mays) and rice (ⅡYou92, Oryza sativa) after exposed in different concentration of rac-metolachlor and S-metolachlor was studied in this paper, trying to evaluate the enantioselectivity behavior of metolachlor in ecosystem.Applicacation of different concentrations of rac-and S-metolachlor posed variable effects on seed germination, the germination was delayed and the shoot length was shorter than that of control. In the comparison of the IC50,5d of root and shoot length of maize and rice after rac-and S-metolachlor treatments, it obviously indicated that root of maize and rice was more sensitivity than shoot. In addition, rice was more sensitivity than maize, the IC50,5d of root and shoot of maize were about 2 and 7-8 times higher than rice. Through investigating the physiological toxicology of maize and rice seedlings, both rac-and S-metolachlor inhibited the elongation of main root and shoot, the growth of lateral roots and root hairs. The toxicology of S-metolachlor was higher than rac-metolachlor at the same concentration. The effect of photosynthesis of maize and rice indicated that the inhibition of the Hill reaction activity clearly increased with increasing concentration, and that treated by S-metolachlor was higher at the same concentration.Under the stress of rac-and S-metolachlor, the root activity of maize and rice were activated at low concentration rac-metolachlor treatment, the activation rate were 14.5% of maize and 22.6% of rice, while the root activity of crops was inhibited by other concentration treatment. The membrane permeability of the roots of crops was increased, the difference was found to be statistically significant of the control. The ultrastructure of rice root tips indicated that rac-and S-metolachlor would destroy the structure of root tip cells, the cell wall was separated from cell membrane, some destroy of nucleus and organella were observed in 3.1μmol/L rac-and S-metolachlor treatment, while the whole cell was destroyed in 12.4μmol/L treatment.By measuring the antioxidant enzymes (SOD, POD, CAT) activity in root of maize and rice, the SOD and POD activity of maize in 18.6μmol/L rac-metolachlor tratment and the POD activity of rice in 1.55μmol/L and 3.1μmol/L rac-metolachlor treatment were activated, other enzyme activities were inhibited. All three enzyme activities in rac-metolachlor treatment was higher than S-metolachlor treatment. In addition, both rac-and S-metolachlor induced the activity of three enzymes in maize leaf cell organelles (chloroplast, mitochondria, and cytoplasm). Except that the POD and CAT activity in chloroplast at rac-metolachlor tratment was higher than S-metolachlor at 18.6μmol/L concentration, other enzyme activities were in the contrary. The result of the MDA content in shoot and root of maize and rice indicated that the MDA content raised with increasing concentration in roots, whereas the MDA content showed a significant increase in low concentration and then decreased in shoot.The pesticide residues after different time were determined by HPLC. The results showed that the dissipation of S-metolachlor was faster than the dissipation of rac-metolachlor at each concentration and that the dissipation rate of rac-and S-metolachlor at low concentrations was faster than that at high concentrations. The half-lives of rac-and S-metolachlor were 169 and 164.8 h at 74.4μmol/L, which were 2.10 times and 2.73 times that at 18.6μmol/L. The metabolites of metolachlor degradation were analyzed by LC-MS. The metabolites of rac-and S-metolachlor in maize root was found to be same, and metabolic pathways and the possible metabolites were estimated. The possible metabolitea were hydroxymetolachlor, deschlorometolachlor, and deschlorometolachlor propanol.