Degradation And Regulation Mechanism Of Malathion And Carbosulfan Pesticide Residues In Pak Choi By Ozone Water Treatment

Spraying chemical pesticides to control pests and diseases is one of the necessary measures to improve crop yield.However,the inappropriate use of pesticides and its high risk to food safety and human health have always been the focus of worldwide attention.Therefore,it is urgent to develop suitable methods for degrading pesticide residues.As a novel treatment form,ozone water can achieve partial degradation of agricultural residues,and has many advantages of broad spectrum,high efficiency,simple operation,lower cost,higher safety,and better guarantee the quality of raw materials.In this research,pak choi（Brassica rapa）,a unique green leafy vegetable in China,and the representative and universal pesticides malathion and carbosulfan,are selected for testing.The research conclusions are as follows:（1）Degradation of pesticide residues in pak choi by ozone waterOn the basis of concentration screening test,2.0 mg/L ozone water is selected to treat pak choi.The contents of two pesticides in pak choi samples are determined through Qu ECh ERS pretreatment,combined with ultra performance liquid chromatography（UPLC）-triple quadrupole（TQD）mass spectrometry.The results show that the pesticides malathion and carbosulfan are degraded to different degrees after 15 min,and the degradation rates are 53.0%and 33.0%,respectively.Based on the first-order reaction of pesticide degradation,a"first+first"-order reaction kinetic model is constructed,with R²>0.9,which can well fit the degradation process of the two pesticides.This research also extensively discusses the degradation differences of the two pesticides.In addition to the k value in the above model,the degradation of pesticides under the condition of ozone water treatment is also related to the factors such as pesticide type,chemical structure,whether they have systemic property,and the pollution degree of pesticide residues.（2）Degradation of pure pesticide solutions by ozone waterThrough gas chromatograph-mass spectrometer（GC-MS）detection,it is found that the degradation rates of pure solutions malathion and carbosulfan after treatment with 2.0 mg/L ozone water for 15 min are as high as 76.9%and 63.1%,respectively.This is significantly better than the above-mentioned effect of degrading the residual pesticides in pak choi.That is,although the leaves of pak choi are thin,the contact area with ozone water is large,and the reaction is relatively sufficient,but there is still matrix interference.At the same time,the new"first+first"-order reaction kinetic model established by this research can still be used to predict the change trend of pesticides,with R²>0.9.（3）Ozonation degradation products and their degradation mechanismsThrough GC-MS analysis and chemical structure analysis,the ozonation degradation products of two pesticides are identified and the degradation processes are analyzed in this research.The oxidation of pesticides by aqueous ozone can occur via direct attack by electrophilic addition and indirect attack by free radicals.Carbosulfan generates intermediate product carbofuran after the nitrogen-sulfur bond（=N-S-）is broken,and carbofuran produces secondary product benzofuranol after hydroxylation.The rupture between the methyne（≡CH）and dimethyl sulphide（DMS）（-S-）,and after hydroxylation,malathion forms succinic acid and phosphoric acid.The reaction may also convert its key structure phosphorothioate（P=S）into a P=O bond by replacing the sulfur atom with an oxygen atom,that is,the emergence of malaoxon.Although some degradation products are generated during ozonation,the concentrations detected by GC-MS are trace and transient,and they are not stable compounds,so it can still be decomposed by ozone water.There is no problem of degradation products,indicating that the application of ozone water to degrade pesticide residues is safe and reliable.（4）Effects of ozone water treatment on the quality of pak choiThe ozonated pak choi has no visible morphological changes,and there are no obvious changes in weight,hardness,color,odor,and taste,but through scanning electron microscope（SEM）observation,it is found for the first time that the guard cells become plump swelling.At the same time,under the premise of basically ensuring the original quality of pak choi,ozone water treatment can also maintain good sterilization and preservation effects（total plate count is reduced by 0.59 lg CFU/g compared to tap water）,delay maturity（soluble solids content（SSC）is reduced by 0.2%）,and improve cell membrane permeability（relative electrical conductivity（REC）is reduced by 2.8%）.However,the content of ascorbic acid（ASA）is decreased by 4.0 mg/100 g.（5）Regulation mechanism of pak choi after ozone water treatmentThrough the RNA sequencing（RNA-seq）and like-targeted metabolomics technologies of plants,the positive feedback effects of pesticide-containing pak choi on the structural constituent and molecule activity of plant intracellular ribosome after ozone water intervention is revealed,and pesticides lead to calcium ion binding and RNA binding.Transcription factors（TFs）such as MYB,AP2/ERF,and WRKY are mainly involved in defense responses,and affect jasmonic acid（JA）metabolic pathway,resulting in the massive synthesis of differentially metabolite JA.In response to reactive oxygen species（ROS）,pak choi produces oxidative stress by plant hormone JA,and also stimulates pentose phosphate pathway（PPP）,glycerophospholipid and glycerolipid metabolism,and promotes flavone and flavonol biosynthesis,so as to protect the tissue morphology of leaves.In addition,photosynthesis is greatly increased and dominated after pak choi is contaminated with residual pesticides,which promotes carbohydrate biosynthesis and provides energy by tricarboxylic acid（TCA）cycle and oxidative phosphorylation.After ozone water treatment,respiration dominates（respiration intensity 22.3 mg CO2/kg·h）,promoting carbohydrate and CO₂ metabolism,and releasing energy by PPP and glycolysis pathways.In addition,the transcriptional and metabolic upregulation of glutathione（GSH）and ATP-binding cassette（ABC）transporters in pak choi caused by pesticide residues,it means that the plant’s own defense mechanisms can reduce the effects of pesticides,and the self-defense of plant is not affected after adding ozone water.