The Inhibitory Effects Of Trans-2-octenal And Trans-2-hexenal On Mango Postharvest Pathogens Of Neofusicoccum Parvum

Mango（Mangifera indica L.）is one of the most important economic crops in the world.Its aroma,juicy and tender meat,sweet and sour taste,and high nutritional value are deeply popular by people.However,the vigorous respiratory metabolism and latent infection of pathogens can cause the decay of mango,which reduces the edible value and commercial value,brings huge pressure on postharvest storage and transportation,severely restricting the development of the mango industry.With the increasing awareness of food safety and the disadvantages of chemical fungicides,using natural and alternative inhibitory is becoming a promising option for control mango postharvest diseases.In this research,pathogens were isolated from naturally decayed mango fruit,and the species of strains were identified by physiological and biochemical experiments combined with molecular methods.The pathogenicity and biological characteristics of the isolated strains were studied to screen the most pathogenic strain.Based on the above experiments,the inhibitory activity and mechanism of essential oils on the most pathogenic strain were explored,and the preservative effect of essential oils on mango fruits was investigated.The main results are as follows:（1）Isolation and identification of mango postharvest pathogenic fungiFour strains were isolated by tissue isolation method.After physiological and biochemical experiments combined with molecular identification,they were identified as Neofusicoccum parvum and Botryosphaeria dothidea,Colletotrichum siamense and Colletotrichum gloeosporioides,the pathogens causing mango stem-end rot and anthracnose,respectively.Pathogenicity and biological characteristics results showed that Neofusicoccum parvum had the strongest pathogenicity,whose optimal growth temperature,optimal growth pH,and spore lethal temperature were 28℃,6.0-8.0,and60℃,respectively.By contrast,those of Botryosphaeria dothidea was 28-30℃,6.0-9.0,and 55℃.In addition,the optimal growth temperature for Colletotrichum siamense and Colletotrichum gloeosporioides was both 28-30℃,but the optimal growth pH and spore lethal temperature was 7.0-9.0,and 55℃,respectively.（2）Effects of essential oils on the mycelial growth of N.parvumThe effects of 10 essential oils on the growth of N.Parvum mycelia were determined by agar dilution method.The results showed that trans-2-pentanal,trans-2-hexenal,trans-2-heptanal,trans-2-octenal,and cuminaldehyde had the best antifungal activity.The minimum inhibitory concentration（MIC）and minimum fungicidal concentration（MFC）were 0.4μL/mL and 0.8μL/mL,respectively.However,cuminaldehyde had a strong and unpleasant smell and trans-2-pentenal and trans-2-heptenal are so expensive that they are not suitable for practical application.Therefore,trans-2-octenal and trans-2-hexenal were selected to apply to mango preservation and the mechanism of these two essential oils on N.Parvum were explored.（3）Inhibitory effects of trans-2-octenal and trans-2-hexenal on N.ParvumResults of in vivo experiments showed that trans-2-octenal and trans-2-hexenal treatments could effectively reduce the incidence of postharvest mango stem-end rot,and their optimal treatment concentrations were 20μL/L and 40μL/L,respectively.Under the optimal treatment concentration,the preservation on fruits without the inoculation of pathogen was evaluated,and the results showed that after 12 d storage,the essential oil treatment could effectively reduce the disease index of mango,prolong the shelf life of 4 d and slow down the decrease rate of V_C.The above treatments were both no adverse effect on mango fruit.（4）Mechanism of trans-2-octenal on N.ParvumThe results of scanning electron microscopy（SEM）showed that the surface morphology of N.Parvum mycelia was changed after the treatment of trans-2-octenal,accompanied by twisting,folding,and shriveling of mycelia.The results of Calcofluor white（CFW）and extracellular alkaline phosphatase（AKP）activity showed that there was no significant difference between the treatment group and the control group within 120 min,indicating that the integrity of cell wall was not destroyed.Further studies showed that the integrity and permeability of N.Parvum cell membrane was changed after trans-2-octenal treatment.However,the content of intracellular MDA and H₂O₂ did not change significantly,indicating that the lipid peroxidation was not occurred.In addition,trans-2-octenal can induce the accumulation of ROS in N.Parvum,resulting in the decrease of mitochondrial membrane potential（MMP）.Meanwhile,ATP content and energy charge decreased,indicating that mitochondrial energy metabolism was affected.Dissolved oxygen experiments verified that trans-2-octenal could significantly influence the respiratory metabolism of N.Parvum,and the superposition ratio showed that trans-2-octenal mainly inhibited the tricarboxylic acid cycle（TCA）.The reduced activities of malate dehydrogenase（MDH）and succinate dehydrogenase（SDH）and the results of molecular docking could further confirm the above view.In summary,trans-2-octenal affected mitochondrial respiratory meta-bolism and energy metabolism,leading to mitochondrial dysfunction and reactive oxygen species（ROS）outbreak.（5）The mechanism of trans-2-hexenal on N.ParvumPI results showed that 1/2 MIC trans-2-hexenal treatment for 30 min could affect the integrity of the cell membrane.FT-IR results showed that nucleic acids,proteins,polysaccharides,and other macromolecular substances in cells were significantly outflowed after trans-2-hexenal treatment.At the same time,a large amount of intracellular ROS was accumulated,indicating the occurrence of lipid peroxidation.After MIC treatment for 30 min,the content both of total lipid and ergosterol were decreased significantly.Essential oil treatment for 60 min destroyed the integrity of cell wall.In summary,trans-2-hexenal passes through the cell membrane depending on its lipid solubility,induces excessive ROS accumulation and membrane lipid peroxidation,affecting the contents of total lipid and ergosterol,enhancing the permeability of cell membrane,and causing the loss of intracellular substances.This study revealed the mechanism of trans-2-octenal and trans-2-hexenal on N.Parvum and applied these two essential oils to the preservation of mango fruits,which can provide theoretical and technical reference for the antifungal mechanism and preservation of essential oils.