Effect Of Exogenous Apple Polyphenols On Quality And Disease In Harvested Litchi Fruit

Litchi(Litchi chinensis Sonn)is a non-climacteric fruit,containing a variety of sugars,vitamins,organic acids,phenols and aroma components with high nutritional value.Litchi is picked at the ripe period,following harvest,litchi fruit undergoes a rapid process of quality deterioration and decay,as manifested by pericarp browning,desiccation,flavor loss and nutrient degradation,which severely restricts the shelf life and marketability.In addition,the disease caused by Peronophythora litchii is one of the most common diseases in litchi.The pathogen causes fruit dropping and decay during preharvest and/or postharvest stages,resulting in huge economic losses.Application of safe and effective natural bioactive products that can inhibit aging and maintain the quality of fruits and vegetables is the one of the recognized preservation methods for consumers,which has become a hotspot researchin postharvest field.In a previous study,we focused on the positive effect of exogenous AP treatment on pericarp browning of postharvest litchi and its mechanism involved in prevention of phenolics oxidation and enhancement of antioxidant enzymes activity.Based on the previous findings,litchi fruit cv.Dadingxiang and P.litchii were used to further study the effect of exogenous AP treatment on quality and disease in harvested litchi fruit and its possible mechanisms.The results are shown as follows:(1)Exogenous AP(5 mg mL-1)treatment significantly delayed the browning process and slowed the change of pericarp color,decreased weight loss and inhibited respiration rate of litchi fruit,delayed the change trend of glucose and fructose in pulp.AP-treated fruit exhibited higher levels of sucrose sugar,organic acids,phenolics,ascorbic acid and reduced glutathione contents in litchi arils during storage.Moreover,AP treatment improved antioxidant capacity,as indicated by DPPH radical-scavenging activity and ferric ion reducing antioxidant power.The results suggest that AP treatment can delay the senescence and quality deterioration and prolong the shelf-life of harvested litchi fruit.(2)Under the AP treatment at concentration of 5 mg mL-1,the inhibition ratio of mycelial grown and dry weight in vitro were 64.25%and 81.63%,respectively.At the concentration of 5 mg mL-1,the disease incidence of litchi fruit inoculated with P.litchii were significantly inhibited.After innoculation of 36 h,the disease incidence of AP-treated fruit was 27.78%lower than that of control fruit.In addition,the lesion expansion and mycelium generation were effectively inhibited by AP treatment.(3)Observed results by optical microscope showed that 5 mg mL-1 AP treatment inhibited the generation of sporangium product,with rate bring up to 94.12%,Also,AP treatment significantly decreased the rate of sporangium germination.Furthermore,the basal of AP-treated germ tubes produced much branches along with reduced mycelial branches,irregular folding and shrunken sporangium.The scanning electron microscopy(SEM)observation results showed that 5 mg mL-1 AP exposure led to much morphological changes of mycelia and sporangium,with the appearance of malformated mycelia,and shrunken,collapsed and accidented sporangium.Transmission electron microscopy(TEM)observation showed that the AP-treated hyphae of P.litchii appeared thickened cell walls with roughed surface.TEM observation also showed that AP treatment resulted in injuried or even destroyed vacuoles and organelles,which consequently contributed to loss of cytoplasm,severe vacuolization and plasmolysis.(4)AP treatment promoted a rapid increase of ROS in sporangium within early stage,with showing a very significant difference at statistic when compared to control.In order to study the effect of AP treatment on plasma membrane permeability,the sporangium was stained using propidium iodide(PI),and the results showed that the integrity of plasma membrane decreased as the treatment time prolonged.Besides,AP treatment led to drastic decline in soluble proteins content in mycelia of P.litchii and caused the degradation of DNA.(5)AP treatment to P.litchii remarkably reduced the reducing sugar content and inhibited the ATPase activity in mycelial cell.With increasing treatment time of AP,the respiration intensity of mycelium was significantly reduced.