Studies On Enantioselective Behaviors Of Chiral Pesticide Metalaxyl In Tobacco

Tobacco is an important cash crop.Metalaxyl,which is widely used in tobacco cultivation,is a germieide against tobacco black shank.Metalaxyl is a chiral pesticide and consists of a pair of enantiomers:(+)-S-metalaxyl and(-)-R-metalaxyl.The enantioselective difference of metalaxyl enantiomers in toxicity,bioactivity,and degradation has been proven.In this study,we established a solid-phase extraction ultra-performance liquid chromatography tandem mass spectroscopy(SPE UPLC-MS/MS)method for determining metalaxyl enantiomers.The degradation of metalaxyl enantiomers during tobacco growth and the process of tobacco curing were studied.The migrations of metalaxyl enantiomers during cigarette smoking were also studied.Incubation experiments were conducted to investigate the effects of metalaxyl enantiomers on the activity of soil enzyme.The main conclusions were summarized as follows:1.The degradations of metalaxyl enantiomers during tobacco growthMetalaxyl was applied by spraying in the field experiments.The results indicated that the degradation rate of(+)-S-metalaxyl was faster than(-)-R-metalaxyl.Half-lives of(+)-S-metalaxyl were 4.9-5.1 d and Half-lives of(-)-R-metalaxyl were 5.7-6.0 d,suggested the degradation behaviors of metalaxyl enantiomers were enantioselective.Metalaxyl was applied by root irrigation in the potted experiments.The results indicated that after root irrigation the concentration of metalaxyl in the leaves increased initially,but declined later.The rate of decline varied depending on the leaf position on the plant: upper leaf > cutter > lower leaf.Enantioselective degradation was not observed in the upper leaves.However,in cutters and lower leaves,significant enantioselective degradation was observed,with a slower rate of degradation of(-)-R-metalaxyl.Finally,the lower the position of the tobacco leaves,the more significant was the enantioselective degradation.2.The degradations of metalaxyl enantiomers during tobacco curingThe degradations of metalaxyl enantiomers during tobacco curing,which is consisted of yellowing stage,color-fixing stage and stem drying stage,were studied.The results indicated that degradation of metalaxyl was promoted by tobacco curing and metalaxyl degraded a lot in color-fixing stage.The degradation rates of metalaxyl enantiomers were almost the same and all EF values remained close to 0.5 during the three stages of tobacco curing.Therefore,enantioselective degradation of metalaxyl enantiomers was not significant during tobacco curing.3.The migrations of metalaxyl enantiomers during cigarette smokingThe migrations of metalaxyl enantiomers during cigarette smoking were studied.The results indicated that the migration rates of(+)-S-metalaxyl and(-)-R-metalaxyl were 4.29%-4.81%,so the migration rates were very low.The residues of metalaxyl enantiomers were almost the same and all EF values remained close to 0.5 before and after cigarette smoking.Therefore,enantioselective migration of metalaxyl enantiomers was not significant during cigarette smoking.4.The effects of metalaxyl enantiomers on soil enzyme activityIncubation experiments were conducted.A pattern of inhibition-recovery-slight stimulation was observed in urease activity of the soil samples treated with metalaxyl enantiomers,but the effects of(-)-R-metalaxyl were stronger than those of(+)-S-metalaxyl at the same concentration.Invertase activity in soil samples treated with metalaxyl enantiomers initially sharply decreased before finally returning to the normal level,and the effects of(+)-S-metalaxyl were stronger than those of(-)-R-metalaxyl at the same concentration.Metalaxyl enantiomers influenced catalase activity in a pattern of slight stimulation-inhibition-recovery,and the effects of(-)-R-metalaxyl were stronger than those of(+)-S-metalaxyl at the same concentration.The results indicated that the effects of metalaxyl on the activity of soil enzymes were enantioselective.