Residues Of Fungicides On Postharvest Tangerine And Tomato And The Mechanism Of EBR Promoting Fungicide Degradation

Pesticide residue problem,a practical issue has been widely existed in China's fruit and vegetable industry.Due to the high pertinence and convenience,chemical fungicide is one of the most used pesticides.It can effectively prevent the rot of fruits and vegetables in the process of transportation and sales,and has an effect of fresh-keeping.However,the application of fungicides in the preservation of postharvest fruits and vegetables might increase the risk of pesticide residues.Meanwhile,the agricultural crops retain toxic residues that may cause serious health complications upon consumption by disturbing metabolic activities.In recent years,most studies have focused on the preservation effect of fungicides,but the distribution and migration of residues in postharvest stages remains unclear.In this context,this study was designed to assess the residual distribution and migration regulations of selected fungicides(imazalil,prochloraz,and thiophanate-methyl)in different parts of the peeled edible tangerine fruit(Citrus unshiu Marc.Cv.Miyagawa).Tomato(Lycopersicon esculentum Mill.Cv.Qian xi),was selected as a representative fruit for direct consumption,and soaked in the recommended solution of boscalid to study the spatial distribution of the fungicide.In this study,24-epibrassinolide(EBR)was used as a detoxicant to promote the degradation of boscalid,and the effect of EBR on the storage quality of postharvest tomato was also investigated.The study's findings are summarized as:1.Regarding distribution and migration of fungicides in tangerine,results showed that imazalil,prochloraz and thiophanate-methyl can penetrate into the interior of tangerine within two hours and residues retained in different percentages in different parts,and the spatial distribution was found as:carpopodium>mesocarp(white cortex+pith)>yellow cortex>pulp.The amount of residues found in the pulp(<0.5 mg kg^-1)was significantly lower than the maximum residual limit(5.0 mg kg^-1)stipulated in China.During the storage,imazalil,prochloraz and thiophanate-methyl showed a certain variation in migration,and the migration path might be"yellow cortex/carpopodium-mesocarp-pulp".Furthermore,the residual amount of imazalil in the whole fruit was:control group?carpopodium-sealed group(7.0 mg kg^-1)>epicarp-sealed group(2.0 mg kg^-1),and the total concentration of imazalil in the interior parts of fruit(mesocarp+pulp)was as follows:control group(1.27 mg kg^-1)>carpopodium-sealed group(1.13 mg kg^-1)>epicarp-sealed group(0.48 mg kg^-1),indicating that yellow cortex and carpopodium were the pathways for imazalil distribution into the different parts of the tangerine,and the yellow cortex was the main route.2.To determine the residual distribution of boscalid and the mechanism of EBR promoting boscalid degradation,the tomatoes were treated with water,boscalid,and EBR+boscalid,respectively.Results showed that boscalid can quickly penetrate into tomatoes within two hours and the residual distribution pattern was found as:peel>pulp>core.Regarding degradation,EBR presented strong effects related to the degradation of boscalid.After 120 h of storage,the residual amount of boscalid in the whole tomato fruit was lower than 3.0 mg kg^-1.EBR promoted the degradation of boscalid with a maximum decrease of 44.8%in 120 h in the peel,and 54.0%and 71.2%in 24 h in the pulp and core,respectively.Additionally,EBR activated the peroxidase(POD),glutathione reductase(GR),and glutathione S-transferase(GST)activities and increased the expression levels of related genes including glutathione reductase(GR),glutathione synthase(GSH1),and glutathione S transferase(GST2),respectively.3.EBR exhibited positive effects on delaying in the process of fruit decay,inhibiting the ethylene releasement,maintaining fruit color and firmness,and increasing the total soluble solids contents.Compared with boscalid treatment,EBR+boscalid treatment significantly reduced the fruit respiration intensity in the early 72 h of storage.However,the recommended dose of boscalid(2000 mg L^-1)tended to cause a certain damage to tomatoes attributed to the increased respiration intensity and relative electrical conductivity.