| Citrus is the world’s largest fruit,with high economic value,fast iteration varieties,good flavor and taste and so on.Citrus fruits are easily infected by a variety of pathogenic fungi during packaging,transportation and storage,among which green mold caused by Penicillium digitatum is an important postharvest disease,which can cause huge economic losses.Paeonol(Pae)is a kind of mono-phenolic compounds with broad-spectrum antimicrobial activity,but its control effect on citrus green mold has not been reported.In this paper,the antifungal effect and mechanism of Paeonol on P.digitatum were studied,and the effects of paeonol on citrus fruit green mold and storage quality were also discussed.The main findings are as follows:1.Inhibitory effect of Pae on P.digitatumPae have strong inhibitory effect on P.digitatum.The minimum inhibitory concentration(MIC)and minimum fungicidal concentration(MFC)of Pae were 0.594g/l and 1.188 g/l respectively.When treated with 0.5×MIC Pae for 30 min,the dry weight of hyphae was only 0.101 ± 0.01 g,which was significantly lower than that of control(P<0.05),and the germination rate of hyphae was only 41.90 ± 1.80 % at 15 h,which was significantly lower than that of control(94.13 ± 0.65 %,P<0.05).After being treated with 0.5×MIC Pae for 30 min,The surface of the mycelium of P.digitatum was twisted and folded,and the deformation was intensified with the extension of time and concentration.After 0.5×MIC Pae treatment for 30 min,the cell membrane was vague,the cytoplasm was not uniform,there were cavities,the material inside the cell leaked out,the endoplasmic reticulum,the nucleus and the liposome degraded.2.Inhibitory mechanism of Pae against P.digitatumPae did not affect the integrity of mycelial cell wall,but it could damage the permeability of mycelial membrane.When treated with 0.5×MIC Pae for 30 min,the content of ergosterol was not affected,but the content of total lipid was decreased and the relative content of unsaturated fat was increased.The content of oleic acid and linoleic acid were 28.40 ± 0.19 % and 40.41 ± 0.32 %,respectively,compared with control group(27.71 ± 0.13 %,36.26 ± 0.10 %,P<0.05).Extracellular conductivity and soluble protein content increased.When treated for 60 min,the mycelium reactive oxygen species(ROS)burst and the mitochondrial membrane potential,ATP content and energy charge decreased significantly,but the AMP and ADP contents were not significantly different from the control group.The inhibition mechanism of Pae to P.digitatum mycelium was that the relative content of unsaturated fat and total lipid were affected,and then the permeability of cell membrane was damaged,and the contents of cell membrane were released.Subsequently,a large number of reactive oxygen species,mitochondrial damage,energy metabolism disorders.Pae showed different inhibitory effects on the spores and mycelium of P.digitatum.Pae can damage the cell wall integrity of P.digitatum spores,but can not damage the cell membrane permeability.After Pae treatment,the cell wall integrity of P.digitatum spores was damaged,and the cell membrane permeability was not affected.When treated with 1.0×MIC Pae for 3 h,the integrity of spore cell wall was impaired.The contents of chitin and β-1,3-glucan were 3.13 ± 0.23 % and 0.89 ±0.00(relative to initial value),respectively,compared with the control group(3.82 ±0.27 %,1.01 ± 0.05 %,P<0.05),the content of chitosan had no significant change.The results indicated that Pae could destroy the cell wall integrity by affecting the contents of chitin and β-1,3-glucan in the spore cell wall of P.digitatum,and then affect the spore germination.3.Induction of Pae on disease resistance of citrus fruit and its control effect on postharvest diseasesPae treatment could improve citrus fruits the scavenging rate of hydroxyl radical and the content of hydrogen peroxide,but had no significant effect on the production rate of superoxide anion radical.5×MFC Pae treated citrus fruits for 1 d,The scavenging rate of hydroxyl radical was 59.58 ± 4.17 %,which was significantly higher than that of control group(49.61 ± 2.58 %,P<0.05),and the content of hydrogen peroxide was 0.62 ± 0.10 μmol/g in 5×MFC Pae treated citrus fruits for 2 d,respectively,it was significantly higher than that of control group(0.37 ± 0.11 μmol/g,P<0.05).The activity of polyphenol oxidase,peroxidase,catalase,phenylalanine ammonia-lyase,β-1,3-glucanase and the content of flavonoids were significantly increased by Pae treatment.5×MFC Pae treated citrus fruits for 1 d,The activity of polyphenol oxidase oxidase was 106.08 ± 22.13 U/g·min,which was significantly higher than that of control group(61.44 ± 15.72 U/g·min,P<0.05).Similarly,the activity of phenylalanine ammonia-lyase and β-1,3-glucanase was significantly higher than that of control group(P<0.05).After 5×MFC Pae treated citrus fruits for 2d,the activity of peroxidase enzyme was 89.28 ± 14.50 U/g·min,which was significantly higher than that of control group(49.61 ± 2.58 U/g·min,P<0.05).After5×MFC Pae citrus fruits for 4 d,the activity of catalase was significantly increased.In addition,Pae treatment of citrus fruit also increased the flavonoid content,but had no significant effect on the total phenol content.As a result,Pae can induce disease resistance in citrus fruits.Pae treatment could reduce the incidence and process of citrus fruit green mold and delay the occurrence of fungal diseases to some extent.In addition,pae treatment had no significant adverse effect on citrus fruit quality.The incidence of P.digitatum in the control group was 100.00 ± 0.00%,and 83.33 ± 5.77% in the 10×MFC Pae treated group,which was significantly lower than that in the control group(P<0.05).At the 5th day,the diameter of disease spot of citrus fruit in 5×MFC Pae was 5.60 ±0.35 cm,10×MFC Pae was 3.88 ± 0.43 cm,which was significantly lower than that in control group(7.00 ± 0.50 cm,P<0.05).After 40 days of natural storage,the natural decay rate of fruits treated with 5×MFC Pae was 5.83 ± 1.44%,10×MFC Pae was3.33 ± 1.44%,which was significantly lower than that of control group(10.00 ±2.50%,P<0.05). |
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