Biochemical Behavior And Enantioselectivity Of Acetanilide Herbicides

Acetanilide herbicides are widely used for preemergence control of broad-leaved weeds. The biochemical behavior of four acetanilide herbicides butachlor, acetochlor, pretilachlor and metoalchlor, and enantioselectivity of metoalchlor have been studied in this thesis.The results showed that the soil catalase and dehydrogenase activities, soil microbial biomass N (B_N) and microbial biomass C (B_C) were restrain somewhat by four acetanilide herbicides at the beginning of cultivation, but can recover soon. Four acetanilide herbicides have little effect on soil ecosystem, and no difference was observed among four herbicides. At the beginning of cultivation, soil enzyme and soil microbial biomass of high concentration was lower than low concentration, but higher than low concentration at the end of cultivation. S-isomer has lower effect on soil ecosystem than rac- metoalchlor. The parameters such as soil enzyme and soil microbial biomass are higher in rice-straw amended treatment than unamended. Rice-straw back to soil can reduce the effect of xenobiotics chemicals.According to the results of the UV difference spectrum curve, the UV difference adsorption reduced when herbicide concentration increased from 0.0 to 1.6μmol.L^-1. UV difference spectra were red-shifted and absorb-peaks became lower. It was shown that herbicide quenched the urease fluorescence mainly through a static quenching procedure. The binding constant and the number of binding site were: acetochlor K= 1.17×10³ L/mol , n=0.81 ; pretilachlor K=1.46×10²L/mol , n=0.67 ; butachlor K = 2.29×10¹L/mol, n=0.50; metolachlor K=1.49×10³L/mol, n=0.84; S-metolachlor K=2.22×10³ L/mol, n=0.8941. The linear relationship between the composition of mobile phase of RP-HPLC and capacity factor of four acetanilide herbicides has been obtained. The excess thermodynamic enthalpy (â–³(H|—)) of acetochor, pretilachor, butachlor and metolachlor were 0.0186 kJ/mol, 0.0256 kJ/mol, 0.0246 kJ/mol, 0.0195 kJ/mol respectively. The relationship between excess thermodynamic function with the binding constant K was suggested.The acute, 24-h LC50 value of rac-metolachlor and S-metolachlor for D. magna was 69.4 and 51.2 mg L^-1 respectively. Both of them were low toxicant based on the classification standard of toxicity. It is demonstrated in chronic test that S-metolachlor is much less toxic to D. magna than rac-metolachlor in low concentration. The data of the chronic toxicity parameters, such as longevity and number of broods per female of D. magna, implied that rac-metolachlor was 10 times more toxic than S-metolachlor. If number of young per female was considered as an evaluation parameter, rac-metolachlor was 100 times more toxic than S-metolachlor. NOEC of rac- and S-metolachlor to D. magna were 0.001 and 0.1 mg L^-1, respectively. Furthermore the environmental factors, such as montmorillonite, humic acid, pH and hardness, all had influence on the toxicity of S- metolachlor on D. magna.V_max of rice cytochrome P450 were as follow: butachlor > pretilachlor > metoalchlor > S- metoalchlor. And so are degradation rate. With the increase of herbicide concentration, the degradation rate decreased. It showed that the metabolism of rice cytochrome P450 is one reason of selectivity of the acetanilide herbicides, and maybe the reason of different effect to kill weed between rac-metoalchlor and S- metoalchlor.