Effects And Mechanisms Of Posthavest Treatments On The Control Of Superficial Scald In Apples

Superficial scald is a kind of serious physiological disorder occurred in apples in coldstorage and is susceptive to most cultivars. It is manifested as brown or black patches on thefruit skin, which damages the fruit appearance and reduces its value. To find the safe andeffective methods to control scald and to further understand its etiology, this thesisinvestigated the effects of chitosan coating, intermittent warming and1-MCP treatments onscald control. From the viewpoint of oxidative stress, the relationships among α-farneseneproduction and oxidation, reactive oxygen species metabolism and antioxidant and their rolesin scald development were also investigated. The aim of this study was to find an alternativemethod for DPA treatment, and to further know the factors influencing scald development,and then to find a whole and effective strategies in scald control. The main results of thisstudy were as follows:（1） Fruit of different maturities was different in scald susceptibility. That is fruit withhigh maturity generally had high scald than those with low maturity. Scald development wasrelated with physiological changes of fruit during the whole storage. Fruit with low maturityhad earlier and higher scald development because of low total antioxidant activity （TAA）,poor capacity in defending oxidative stress and high accumulations of H₂O₂and MDA. Fruitwith high maturity had later and lower scald incidence because of high TAA, delayed loss ofequilibrium between the production and scavenging of reactive oxygen species （ROS） andslowly increased H₂O₂and MDA contents. Scald development could be obviously dividedinto two stages, one stage where oxidative damages accumulated in cold storage and anotherwhere scald symptom appeared quickly in warm temperature. Scald incidence in warmdepended on the degree of oxidative damage in cold. H₂O₂and malonaldehyde （MDA）contents were related with scald development. Polypropylene （PPO） was also correlative, butnot the critical factor.（2） Chitosan coating could effectively control the superficial scald in apples by inhibitingthe respiration rate and the ethylene production, reducing the production of α-farnesene andthe accumulation of conjugated trienols, weakening membrane lipid peroxidation, keeping the integrity of cell membrane, and inhibiting the increase of PPO activities. Chitosan of2%wasthe best in scald control with incidence and severity66.2%and84.8%lower than untreatedfruit.（3） Intermittent warming stimulated ethylene production, which led to higher α-farneseneand conjugated trienols （CTols） accumulations in early storage than in late storage.Intermittent warming improved fruit antioxidant activities obviously higher in early storagethan in late storage, which resulted in higher capacities in defending oxidative stress relatedwith CTols and H₂O₂accumulations and accordingly lower oxidative damage and scaldincidence. Fruit warmed for5days after4weeks cold storage had the lowest scald incidenceand severity, which was85%and61%lower than untreated fruit. Intermittent warminginhibited scald development by reducing the chilling induced oxidative damages.（4）1-MCP markablely improved the total qualities of apple fruit.1-MCP delayed thedecline of fruit firmness and contents of total soluble solids and total acids, inhibited thedevelopment of superficial scald and flesh browning and keep higher levels of concentrationsof total phenolic, flavonoid, ascorbic acid and glutathione and total antioxidant activities,though the effects was less evident on flesh than peel. In fruit peel, TAA was positivelycorrelated with total phenolic, flavonoid, ascorbic acid and glutathione, but in flesh, TAA wasonly positively correlated with total phenolic. Scald development was related with allantioxidants and Tasso TAA value could be used in judging the sensitivity of scald in ‘Fuji’apples.（5） Effects of1-MCP and ethephon treatment on α-farnesene production and oxidation,H₂O₂accumulation and antioxidant capacity depended on fruit maturity. After1-MCPtreatment, less mature H1fruit had lower TAA than untreated ones. However, CTol contentswere still lower in1-MCP treated fruit, because of inhibition of α-farnesene production by1-MCP.1-MCP treated H2fruit had lower CTol accumulations due to the combination oflower α-farnesene production and higher TAA, compared with untreated fruit. Ethephonstimulated the ripening in H1fruit and with higher TAA, α-farnesene production andoxidation were inhibited. However, ethephon promoted α-farnesene production and oxidationin H2fruit, as well as scald development. Scald development is unlikely to depend solely onany single factor–the production and oxidation of α-farnesene, the accumulation of ROS, orantioxidants. More likely, oxidative stress leading to scald occurrence mainly depends on acombination of production and degradation of free radicals related to CTol accumulations. Asthe potential sources of oxidative stress, inhibition of production and oxidation of α-farnesenewas more critical than defending oxidative stress after it has been initiated.（6） Delayed1-MCP treatment lost its effect on controlling fruit ripening and scalddevelopment, which depended on the duration from harvest to treatment and cultivars. Multiple1-MCP treatment did not show additional effects on fruit with1-MCP applicationimmediately after harvest. However, for fruit with delayed treatment, multiple treatment keptfruit quality and controlled the disorders, although the effects determined by the time durationfrom first treatment to repeated treatment. Multiple treatments ensure the1-MCP technologyin keeping fruit and inhibiting disorders and have great commercial values.