Oxidative Damage And Genetic Toxicity Effects Of Metalaxyl-M On Earthworm(Eisenia Fetida)

Metalaxyl-M is the first commercial fungicide with stereo optical activity and has a notable effect on the prevention and control of crop-diseases which caused by Peronospora spp., Phytophthora spp., Pythium spp., among others. Metalaxyl has two kinds of enantiomers, which are S-enantiomer and R-enantiomer. Moreover, R-enantiomer plays an important role in the activity of metalaxyl. However, the traditional metalaxyl was sold as racemic formulations(rac-metalaxyl). Metalaxyl-M is mainly the R-enantiomer of metalaxyl, which typically comprises 97.5% of the R-enantiomer and 2.5% of S-enantiomer. At present, the traditional metalaxyl(rac-metalaxyl) has been already replaced by metalaxyl-M, and widely used all over the world.Metalaxyl-M, as a kind of fungicide, will eventually be released into the environment. Therefore, we must consider the impact of metalaxyl-M on people and environment. However, the current research emphasis of metalaxyl-M all focusses on its degradation. The environmental risk assessment of metalaxyl-M is mainly based on the data of rac-metalaxyl. Studies have shown that different enantiomers had different behaviors in the environment. The activity of R-enantiomer is 3-10 times higher than that of S-enantiomer. Therefore, we suggest that the toxicity of metalaxyl-M may be higher than that of the rac-metalaxyl. Study has reported that the toxicity of R-metalaxyl to aquatic organisms was higher than that of racmetalaxyl. Nonetheless, there was no sufficient data concerning the toxic effects of metalaxylM on soil organisms. Therefore, it is very necessary to supply a series of data on the toxic effects of metalaxyl-M on soil organisms. Therefore, in the present study, the earthworm(Eisenia fetida) was selected as the model organism to study the toxic effects of metalaxyl-M on antioxidant enzyme systems, ROS and DNA damage in earthworms. The purpose of the present research was to verify whether metalaxyl-M has cytotoxic and genotoxic effects on earthworms. Moreover, it may supply the data about the toxic effects of metalaxyl-M on soil organisms and provide a theoretical basis for an evaluation of the environmental safety of metalaxyl-M. The main conclusions of the present study were as follows:(1) The ROS level increased substantially with increasing concentration. The ROS level of different metalaxyl-M-exposed treatments was all high than that of the control during the exposure period, except at 1 mg kg-1 on the 7th day. Significant differences were observed when the concentration was higher than 0.1mg kg-1 during the exposure period.(2) In general, the MDA content increased firstly and decreased subsequently during the exposure period. There were obvious enhancements at 1 and 3 mg kg-1 on the 7th day. On the 14 th day, only 3 mg kg-1 metalaxyl-M-exposed treatment’s MDA content was significantly higher than that of the control. The MDA content decreased on the 21 st and 28 th days, and there were almost no significant differences among different metalaxyl-M-exposed treatments.(3) The comet assay was the most sensitive indicators in the present study. During the exposure period, the OTM increased greatly with increasing concentration and time. Significant differences were observed when the concentration was higher than 0.1 mg kg-1. Compared with the control, the OTM of 0.1, 1, 3 mg kg-1 metalaxyl-M-exposed treatments increased 5, 7, 12 times on the 28 th day.(4) The SOD activity was greatly enhanced with increasing concentration on the 7th day. Nonetheless, the enhancement disappeared on the 14 th, 21 st and 28 th days. On the contrary, the SOD activity was significantly inhibited with increasing concentration on the 14 th, 21 st and 28 th days. In addition, the SOD activity also decreased with increasing time during the exposure period. In general, the CAT activity kept growing during the exposure period, which showed a similar trend to the ROS level. The CAT activity was greatly enhanced when the concentration was higher than 0.1 mg kg-1. In addition, the CAT activity of 1mg kg-1 treatment group significantly increased on the 7th and 21 st days and then decreased on the 14 th and 28 th days, which presented the trend of dropping first, then rising, and finally dropping during the exposure period. In general, the POD activity had no obvious trend during the exposure period expect on the 7th day, which showed a similar trend to the MDA content. On the 7th day, the POD activity was greatly enhanced with increasing concentration. On the 14 th day, only the 3 mg kg-1 metalaxyl-M-exposed treatment’s POD activity had significant difference compared to that of other treatments. Although the POD activity of different metalaxyl-Mexposed treatments had little difference on the 21 st day, they showed obvious difference on the 28 th day. The POD activity of 0.1 mg kg-1 treatment was higher than that of other treatments on the 28 th day.(5) Although GST activities appeared to an upward trend with increasing concentration on the 7th and 14 th days, this promotion disappeared on the 21 st and 28 th days. On the 7th day, an obvious difference was observed at 1 mg kg-1. On the 14 th day, the difference became more obvious than that on the 7th day. On the days of 21 and 28, there was little difference in the GST activity among different metalaxyl-M-exposed treatments.