Study On Antifungal Mechanism Of The Loquat Leaf Extract Against Green Mould And Effcets On Citrus Fruit Preservation

Citrus industry plays an important role in economic development of China.The green mold disease caused significant economic losses in citrus and the overuse of traditional chemicals about it may leads to fungicide resistance.In recent years,in order to break the dilemma,the extraction of natural antifungal agent from plants to replace the traditional fungicides has gradually become a research hotspot.Loquat leaves as a well-known Chinese herbal medicine,there are few reports on the application of loquat leaves in the preservation of fruits and vegetables,and the antifungal mechanism is little known.In this study,the antifungal mechanism of loquat leaf extract on Penicillium digitatum was studied,and the effect of compound coating of loquat leaf extract and sodium alginate on the preservation of citrus fruit was analyzed.The purpose of this paper is to provide theoretical support for the prevention and control of citrus green mold and the development of antifungal agents for citrus.The main results are as follows:1.Taking P.digitatum as the target fungus,the antifungal effect of loquat leaf extract on isolated pathogens was studied.The results showed that the minimum inhibitory concentration（MIC）and minimum fungicidal concentration（MFC）of loquat leaf extract against P.digitatum were 0.625 mg/m L and 1.25 mg/m L,respectively.The inhibitory effect of loquat leaves had dose effect on P.digitatum,and it could inhibit the germination of P.digitatum spores,destroy the normal morphology of spores and inhibit the growth of hyphae.Fluorescence staining also showed that P.digitatum was damaged more severely under the action of loquat leaf extract than the normal.In addition,after the treatment of loquat leaf extract,the cell lysis rate rose,the intracellular electrolytes and biological macromolecules were leaked into the supernatant,the activity of cell wall hydrolase increased,the activity of alkaline phosphatase（AKP）enzyme rose,and the content of malondialdehyde（MDA）in mycelium increased,while the content of total lipid and ergosterol decreased.The results showed that loquat leaf extract of loquat leaves could damage the structure and function of cell wall and cell membrane of P.digitatum.In the process of energy metabolism,loquat leaf extract could reduce the contents of ATP,ADP and AMP in mycelium,and weaken the enzyme activities of succinate dehydrogenase（SDH）and malate dehydrogenase（MDH）in tricarboxylic acid cycle,three kinds of ATP enzyme and cytochrome C oxidase（CCO）.The antifungal mechanism of loquat leaf extract on P.digitatum was elucidated in the aspects of structure and function of cell wall,cell membrane and energy metabolism.2.The wound inoculation study on Xinyu mandarin was used to determine the content of disease resistant substances,enzyme activity and relative gene expression in the pericarp.The results showed that loquat leaf extract could reduce the incidence of Xinyu mandarin inoculated with P.digitatum,reduce the enlargement of plaque diameter,effectively delay the decrease of pericarp hardness and lignin content,the accumulation of MDA and hydrogen peroxide（H₂О₂）content,and maintain the ascorbic acid（As A）content in the pericarp at a high level.Loquat leaf extract could inhibit the degradation of antioxidants such as total phenols and flavonoids,induce a variety of resistance defense enzymes in the pericarp to a certain extent and promote the up-regulated expression of related genes to achieve the effect of improving fruit resistance to green mold.3.The loquat leaf extract was mixed with sodium alginate and other substances to make a preservative agent to study on Nanfeng mandarin preservation.During storage,the compound coating with loquat leaves could effectively reduce the weight loss rate and decay rate,weaken the respiration,delay the decline of soluble solids（TSS）,As A,titratable acid（TA）and the change of color to maintain the marketability of Nanfeng mandarin.In addition,by weakening the accumulation of MDA and hydrogen peroxide,increasing the contents of total phenols and flavonoids in pulp,and inducing the increase of antioxidant enzymes and disease defense enzymes such as peroxidase（POD）,superoxide dismutase（SOD）,ascorbate peroxidase（APX）and catalase（CAT）,the senescence of fruit was delayed and the storage period of citrus fruit was prolonged.It shows that the compound coating with loquat leaves has great potential in citrus preservation.