Research On The Enantioselective Ecotoxicity Of Rac-metolachlor And Its S-enantiomer

Widespread application of chiral herbicides is inevitable to occur into the environment.In general,the enantiomers of chiral pesticides are different in biological processes,therefore,resulting enantioselective ecotoxicity to nontarget biology.Nowadays,the environmental safe behaviour and enantioselective ecotoxicology of chiral herbicides have become a new point of global scientific research.Metolachlor is one of the widely used chloroacetanilide herbicides,which contains two R-isomers and two S-isomers.S-enantiomer is more effective in the herbicidal activity while R-enantiomer was reported having a mutagenic effect.In this paper,the plant physiological effects,enzyme activities and cell ultrastructures of maize(Yedan13,Zea mays) and rice(â…¡You92,Oryza sativa) after exposed in Rac-metolachlor(Racmt) and S-metolachlor(Smt) were researched.The enantioselectivity of metolachlor was tried to evaluate in these two widely cultivated crops.In addition,due to the overuse of metolachlor,herbicides are easily transformed to the surface water or ground water and accumulated in the aquatic organisms,may deteriorate the balance of aquatic ecosystems.In order to well understand their ecological risks and enantiomer selectivity in aquatic system,the acute toxicity of Chlorella pyrenoidosa as well as catalase activity and cell ultrastructure after Racmt and Smt exposure were researched.In the comparison of the EC₅₀ of maize and rice after Racmt and Smt treatments, it obviously indicated that rice was more sensitivity than maize,for the EC_50,96h of rice were 12.32μM of Racmt,9.44μM of Smt.However,the EC_50,96h of maize were 782.73μM,755.98μM,respectively,which were about 60 times more than rice.The elongation of main root,growth of lateral root and shoot of maize and rice seedlings were inhibited,the chlorophyll a,chlorophyll b and total Chlorophyll were reduced after Racmt and Smt treatments.The antioxidant enzymes(POD,SOD),CYP2E1, GSH and GST activity were induced after herbicide exposure.Besides that the SOD activity in Racmt treatment was higher than Smt treatment,other enzyme activities were in the contrary.The ultrastructure of rice seedlings indicated that Racmt and Smt would destroy the chloroplast and mitochondria,make chloroplast lamellar structure sparseness,reduce the content of chlorophyll,disorder mitochondrial cristae,even lead to vacuity of mitochondria.Some minor expansion in the rough endoplasmic reticulum and a slightly swelling in the end of the Golgi complex were observed,all of these phenomena may indicate that biological molecular synthesis or transportation were blocked.The toxicity of the chiral herbicides Racmt and Smt to Chlorella pyrenoidosa was determined and compared in this study,based on four different test endpoints:the growth inhibition rate,the chlorophyll a and chlorophyll b concentration,the catalase activity,and the ultrastructural morphology of cells.The 24h,48h,72h and 96h EC₅₀ values of Racmt were 0.196,0.241,0.177 and 0.152 mg L^-1,respectively,these values were higher than those of Smt,which were 0.116,0.106,0.081 and 0.068mg L^-1,respectively.This indicates that Smt was more toxic to Chlorella pyrenoidosa than Racmt.The Ch1 a and Ch1 b concentration of Chlorella pyrenoidosa treated by Racmt was higher than that treated by Smt.In general,the catalase activity of Chlorella pyrenoidosa treated by Smt was higher than that exposed to Racmt,and catalase activity was inhibited at high concentrations of both herbicides.The ultrastructural morphology of cells grown in the two herbicides was observed by transmission electron microscopy.The cell wall separated from the cell membrane, accumulated starch granules were observed in the chloroplast,and some lipid droplets and unknown electron-opaque deposits were also observed in the cytoplasm.There were some dead cells under separation after herbicide exposure,especially in the cells of Smt treatment.These results indicated that Racmt and Smt had selective toxic effects to Chlorella pyrenoidosa.Both of them destroyed the cell structure,deterred the normal cell growth and metabolism.