Enantioselective Toxicology Of Chiral Herbicide Diclofop On Rice And Blue-green Alga

Aryloxyphenoxypropionic herbicides (AOPP) are systemic and post-emergence herbicides that were introduced in the 1970s. Most of them are synthesized and applied in their racemic forms nowadays, and many studies have reported the risks of exposure to AOPP for birds, mammals, freshwater fish, invertebrates and terrestrial plants at the racemic level. However, there is little information on either the toxicity of this kind of herbicide to non-target aquatic plants or the environmental behaviors of AOPP at the chiral level. Studies on the enantioselective ecotoxicology and environmental behavior of chiral herbicides are necessary, because these studies are important in chiral pesticide application and environmental risk assessment.Our studies focused on the enantioselective eco-effects of chiral herbicide diclofop on nontarget plants including rice and blue-green alga. Rice is a nontarget beneficial plant model while blue-green alga is a nontarget harmful plant model. The EC50 values and Hill reaction activities of rice xiushui 63 seedlings exposed to chiral diclofop acid were tested. The morphological response, ultrastructural variation, oxidation damage and peptide toxin of blue-green alga Microcystis aeruginosa exposed to chiral diclofop were tested. Furthermore, the enantioselective degradation of diclofop acid in rice culture medium was conducted.The acute toxicity (72-h EC50 values) of chiral diclofop acid towards rice Xiushui 63 seedlings and its effects on the Hill reaction activities of chloroplasts were determined. Significant differences were observed between the two enantiomers in 72-h EC50 values and in both in vivo and in vitro relative Hill reaction activities. These observations indicated that the enantiomers of diclofop acid pose different toxicities to rice seedlings:the S-enantiomer is more toxic to leaves and the R-enantiomer is more toxic to roots.Rac-DC, R-DC and S-DC exhibited different properties in degradation. Degradation of rac-DC fits second-order reaction kinetic analysis, and R-DC and S-DC fit zero-order reaction kinetic analysis. There is seldom difference among different concentrations of the chemicals. A connection between degradation property and toxicity of the chemical may exist.Under the laboratory conditions, rac-DC, R-DC, S-DC and DM stimulated the biomass of Microcystis aeruginosa apparently. However, the protein content increased at first and decreased thereafter. The color of the alga changed from blue-green to green. We indicated preliminary that this stimulation was non-positive. The increase in biomass may be attributed to unnormal cells. Morphological response of Microcystis aeruginosa indicated the toxicity of diclofop. Colony can be potent in resisting environmental pressure and increase the livability of the cells. Ultrastructure study of Microcystis aeruginosa indicated the variations in cell-organelle when exposed to diclofop. Programmed cell death (PCD) occurred. The toxicities of the chemicals were rac-DC＞DM＞R-DC＞S-DC. These results demonstrated enantioselectivity between the two enantiomers.For other three AOPPs, the non-consistence between cell density and protein content were also observed. AOPPs posed great effects on nontarget blue-green alga in the environment. Enantioselectivity exists between two enantiomers, and the attention and research are needed.From the results of ROS and MDA, we found that diclofop posed oxidation pressure on Microcystis aeruginosa. But there was no damage observed in the first two days from the results of SOD, because oxidation damage was related to the concentration and exposure time of the chemicals. When the increasement of oxygen interrupts anti-oxygen system, the bio-process of cell will be damaged, and PCD occurred. Under the same concentration (5mg/L) of the chemicals, rac-DC is the most toxic, followed by R-DC, S-DC and DM. Rac-DC induced PCD and necrosis, and other three chemicals induced PCD.The release of Microcystin-LR was affected by different forms of diclofop. R-DC was more potent in stimulating MC-LR than S-DC.Combining the efficient herbicidal form of diclofop and the results in our studies, we found that although R-DC is herbicidal, it poses more potent effects on both rice and blue-green alga. Use R-DC only may not be the best choice today because the eutropbication of the surface water occurs oftenly. Proper application of pesticides especially chiral pesticides should gain more attention not only in crop production but also in environmental protection. Residues of pesticides in the environment can bring huge burden. Studies on the ecotoxicology at chiral level can make contribution in the environmental risk assessment.