Effects And Molecular Basis Of UV-B And Visible Light On The Occurrence Of Superficial Scald In Apples

Superficial scald is a physiological disorder of apple fruit during cold storage,it mainly occurs in the epidermal cells of the apple fruit,presents brown spots when it is affected,and forms into slices in severe cases,which seriously affects the fruit quality and commercial value.Bagging is often used in apple farming to prevent fruit pests and diseases in China.Recent studies have shown that pre-harvest light can affect the occurrence of superficial scald in pear fruit,but the mechanism of the occurrence of scald caused by light and the direct pathogenic factors of scald have not been systematically study.Therefore,it is of great theoretical and practical significance to study the effect of light especially UV-B and visible light on superficial scald of apple and the mechanism.In this study,red skin cultivar‘Fuji’（Mdlus domestica Borkh c.v.‘Fuji’）and‘White winter pearmain’（Mdlus domestica Borkh c.v.‘White winter pearmain’）with green skin as the experimental material,the effect of UV-B and visible light treatment on scald development of apple peel were studied,and the pathogenesis of superficial scald were explored.This study provides a theoretical basis and a new strategy for elucidating the cause of fruit scald,and finding effective and environmentally friendly methods to control superficial scald.The main results obtained are as follows:（1）There were no significant effects on the ethylene production rate,respiration rate,fruit firmness and soluble solids content of apple fruit under unbagged,and UV-B-,and visible light-treatment.However,bagging affected fruit coloration.The study results showed that long time UV-B and visible light treatment increased the coloration of‘red Fuji’apple,while in‘White winter pearmain’apple,only UV-B promoted the synthesis of red color,and the visible light had no obvious effect.（2）No matter the red apple or the green apple,there was almost no scald on the blushed peel of the unbagged fruit,while there was the occurrence of scald on the fruit shaded peel,these results suggested that light could inhibit the occurrence of scald.The scald of the‘red Fuji’apple was aggravated after bagging,but the scald on the‘White winter pearmain’apple was catabatic.These results shown that the scald incidence of red and green apple was opposite by bagging treatment.Using the visible light and UV-B to irradiation bagged fruit,it was found that the visible light and UV-B treatment significant inhibited the occurrence of scald.These results shown that the occurrence of scald were significant inhibitted by visible light and UV-B treatment in postharvest red and green apple fruit.（3）In the late of occurrence of superficial scald in the red apple or the green apple,there was no correlation between the accumulation of conjugated trienols（CTols）,the oxidation product ofα-farnesene and the occurrence of superficial scald in different treatments.Therefore,the accumulation ofα-farnesene and CTols were not the factors that directly lead to the occurrence of superficial scald.（4）In order to further study the cause of the occurrence of superficial scald in apple,we measured the scald peel and unscald peel.The results showed that the accumulation of phenolic substances was higher in scald peel than the unscald peel.We added the solution of catechin and chlorogenic acid into the unscald peel,then the peel turned brown,which the phenotype was similar to scald peel.These results suggested that superficial scald may be a browning reaction occurring on the fruit peel,which was closely related to the oxidation of phenols into quinone.（5）In this study,we found that socking reductive substances or injecting anti-sense MdPPO expression vector could significantly inhibited the occurrence of scald in apple,while the injection of the PPO protein aggravated the occurrence of scald.The above results indicated that the occurrence of superficial scald was closely related to the process of polyphenol oxidase（PPO）catalyzing the reaction of phenolic substances and reactive oxygen species（ROS）to form quinone.（6）Our results showed that the ROS and PPO were significantly increased in the blushed of unbagged fruit,and the ROS was also significantly increased on the blushed of visible light and UV-B treated fruit,but the fruit did not develop superficial scald.In addition,our study also found that overexpression the gene vector of anthocyanin synthesis（MdbHLH3expression vector）obviously inhibited the occurrence of scald.And adding the reductive substance could obviously alleviated the fruit browning,after soaking the scald peel with reductive substance,the peel brown significantly lighter because quinone could react with reductive substance to form colorless phenols.Therefore,the accumulation of quinone was affected by the oxidation of phenols and the reduction of quinone.The more ROS,the easier the phenolic substances were to be oxidized to quinone,and the stronger the total antioxidant capacity（TAC）,the easier the quinone was to be reduced to colorless phenols.Therefore,the ratio of H₂O₂/TAC was used to reflect the fruit’s tendency in the dynamic process of phenolic oxidation and quinone reduction.The higher of the H₂O₂/TAC ratio,the more quinone accumulation tends to occurrence of scald in the peel.Our results showed that the incidence of the scald with high H₂O₂/TAC ratio was higher than the peel with low H₂O₂/TAC ratio,except for the blushed fruit.These results also indicated that the occurrence of scald was closely related to the accumulation of quinone in the peel.（7）PPO is located in the plastid,phenolic substance is distributed outside the plastid.Therefore,when the plastid membrane is not damaged,even if phenolic substances,ROS and PPO are accumulated,but the quinone will not be accumulated.Our results showed that the waxy structure and the cell structure were intact,and the plastid membrane was not damaged.Although the ratio of H₂O₂/TAC was higher on the blushed peel of the unbagged fruit,the fruit did not suffer from scald.（8）When visible light and UV-B irradiate apple,we found that UV-B could significantly induce the coloration of‘White winter pearmain’apple fruit,while visible light could not.Through transcriptome and metabolomics data,this study showed that UV-B and visible light could significantly induce the expression of genes related to the synthesis of flavonoids and anthocyanins and the accumulation of metabolites.In particular,UV-B treatment significantly increased the content of anthocyanin 3,5-diglucoside,thus promoting the coloration of fruit.Anthocyanin as an antioxidant that could significantly inhibited the occurrence of scald.Therefore,artificial UV-B treatment could effectively prevent the occurrence of scald in green apple.In summary,our study demonstrated that bagging has a distinct effect on the occurrence of scald of red and green apple.Nature light,UV-B and visible light treatment could inhibite the occurrence of scald.In addition,the occurrence of scald was closely related to the accumulation of quinones.Reducing the oxidation of phenols or increasing the reduction of quinones could effectively alleviate the occurrence of scald.In addition,the effect of UV-B on the fruit coloration of green apple varieties was studied,which provided a theoretical basis for further exploring the genes related to anthocyanin synthesis.