Research On Mefenacet Degradation In Paddyfield And The Influence On Rice Rhizosphere

Mefenacet is the herbicide that is widely used in transplanted rice to control monocotyledonous weeds and some dicotyledonous weeds.However,a large amount of mefenacet can be adsorbed in soil for a long time and the degradation products also have toxicity to paddy field.Thus,mefenacet may pose a potential threat to the soil environment and affect the growth and development of current crops or succeeding crops.The rhizosphere microorganism is the communication closest microbe to the root,and it plays critical role in plant growth and development through chemical communication,such as plant hormones and volatile organic compounds,which directly or indirectly promote plant growth and regulate plants to enhance the stress resistance.In this paper,16S rDNA sequencing combined with metabolomics was used to investigate the changes of rice,rhizosphere microorganisms and rhizosphere metabolism under the exposure of mefenacet,which may provide new ideas for the study of herbicide tolerance and self-healing system of plants.1.Identify the metabolites of mefenacet.In hydrolysis experiment,the hydrolysis products of mefenacet was studied by QuEChERS extraction method and HPLC-Q-TOF/MS.The established method is stable and reliable.The hydrolysis degradation products of mefenacet were selected and identified as2-benzothiazoloxy acetic acid and N-methylaniline.2.Establish a method for the determination of mefenacet and its metabolites.The study optimized the content of formic acid and NH₃·H₂O in the extraction solvent.HPLC-MS/MS was used to establish a method for the simultaneous determination of mefenacet and its metabolites,and the method was verified.The results showed that the matrix effects,recoveries of the target compounds were in the range of-18.68%-10.48%and 80.98%-118.44%.The limits of quantitation of mefenacet,N-methylaniline and 2-benzothiazoloxy acetic acid were 0.1μg/kg、1μg/kg and 1μg/kg.3.Investigate the migration and transformation of mefenacet in rice field.The results showed that the rice field had the ability to enrich mefenacet and the enrichment ability was:soil>root>shoot>water.The distribution of mefenacet content in soil reached the peak at 3 days after application.The rice had a certain absorption effect of mefenacet,which was transported from the root to the shoot.At the same time,it was found that mefenacet could produce more 2-benzothiazoloxy acetic in black soil.The metabolite 2-benzothiazoloxy acetic appeared in paddy field environment two hours after application of the treatment with a continued increase.The extractable content of metabolite N-methylaniline decreased over time once released to the soil.It was easy for N-methylaniline to form binding residues in soil and there existed adsorption saturation of high concentration N-methylaniline in soil.4.Investigate the effect of mefenacet on the rhizosphere interaction of rice seedlings.Comparing the growth and root phytohormones of soil-cultured and water-cultured rice seedlings,it was found that the apparent growth of soil-cultured rice seedlings had no significant toxic effect,while the growth of water-cultured rice seedlings was significantly inhibited.The hormones content results showed that indole-3-acetic acid and salicylic acid in soil-cultured rice roots was significantly higher than that in the control while indole-3-acetic acid in water-cultured rice roots had no significant increase.However,water-cultured rice roots had a higher content of salicylic acid and abscisic acid.16S rDNA sequencing method was used to investigate the effect of mefenacet on rice rhizosphere microorganism.It was found that low application of mefenacet had little effect on the rhizosphere microbial structure while high application of mefenacet increased theα-diversity of microbial community and the abundance of Bacillus.HPLC-Q-TOF/MS was used to investigate the effect of mefenacet on rhizosphere metabolism.It was found that the treatment of mefenacet enhanced the basic metabolisms of rhizosphere microorganism and the hormone synthesis pathways,such as salicylic acid synthesis and auxin synthesis.Therefore,under the stress of mefenacet,phytohormones regulate the growth of rice and rhizosphere microorganisms improve the mefenacet tolerance through adjusting phytohormones,such as the enhancement of tryptophan metabolism by Bacillus.This conclusion provides us new ideas for the study of plant herbicide tolerance.