Effect Of Fruit Maturity On The Chilling Tolerance Of Postharvest Cucumber And The Physiological Mechanism Involved

The harvest maturity stage of horticultural product can influence its postharvest quality and stress response, so it is always be the focus of agricultural practice and utilization of product. This research study the effect of fruit maturity on the chilling tolerance of postharvest cucumber and the physiological mechanism involved, the result would greatly benefit the instruction of agricultural cultivation and postharvest fruit utilization of cucumber.Cucumber fruits exhibit a single-sigmoidal growth curve. Based on fruit size and color, four distinct developmental stages were chosen:Immature (3-8DAA), Mature (9-16DAA), Breaker (17-22DAA) and Yellow (35-40DAA). The chilling tolerance of cucumber fruits was remarkably affected by the fruit maturity. Fruits at earlier developmental stages are more susceptible to chilling stress. So storing cucumber fruits with advanced maturity would alleviate chilling injury and extend post-harvest life at low temperate.Increased oxidative stress may be an intrinsic feature of the development and senescence of cucumber fruits. The increase in H2O2-generating enzyme (SOD) activity and the decrease in the activities of the two main H2O2-scavenging enzymes (CAT and APX) may cause the accumulation of H2O2during cucumber development and senescence. The decline in GSH content and increase in ASC content during fruit maturation suggest that GSH is important for cell division during the formation of cucumber fruits, whereas ASC is critical for cell expansion during their development and senescence. Significant increases in POD mRNA level, activity, and ASC content are conspicuous features that characterize the development and senescence of cucumber fruits, reflecting the close correlation between POD, ASC, and cucumber fruit maturation.The development of chilling injury is related to increased oxidative stress under chilling conditions. SOD showed an early response to chilling stress but became inefficient with the development of chilling injury in all fruits. CAT and APX appear less effective in scavenging the H2O2produced under chilling stress. The recycling activity of ASC may be essential to the maintenance of the ASC/DHA ratio, and the higher ASC redox state might prevent oxidative damage in more mature cucumber fruits. Significant higher POD activity and ASC content may provide a higher antioxidant capacity and achieve stronger chilling tolerance for fruits at later developmental stages.Total sugar content and reduced sugar content showed little change during the development of cucumber fruit, and both significantly declined in Yellow fruit. Sucrose, glucose and fructose content all decreased during the development and senescence of cucumber fruit. The activity of SS and SPS increased during fruit maturation, but the mRNA level of SS declined. The organic acid accumulated during the growth of cucumber fruit, the content of chlorogenic acid and total oxalic acid increased, but the content of tannic acid, gallic acid and water soluble oxalic acid declince. This sugar-acid composition constitutes the special flavor of cucumber fruit.Total sugar content and reduced sugar content in all fruit slightly increased during cold storage, but both notablely declined after rewarm. Sucrose content increased in Immature and Mature fruit during cold storage and declined after rewarm, but decreased in Breaker and Yellow fruit during cold storage and increased after rewarm. Glucose and fructose content showed little change during cold storage, but increased after rewarm except declined in Yellow fruit. SS and SPS activity both slightly increased in Maturee and Breaker fruit during chilling stress, whereas complement to each other in Immature and Yellow fruit. SS activity may directly influenced by SS transcription level. Organic acid content increased in cucumber fruit under chilling stress. During cold storage analysed organic acid in cucumber fruit showed little change, only gallic acid content declined in Immature fruit. Organic acid content in cucumber fruit increased after rewarm.During the development and senescence of cucumber fruit PG and pectinase activity showed little change, while both significantly increased in Breaker fruit, and pectinase activity declined remarkablely in Yellow fruit. Both cellulase and β glucosidase activity declined during the growth of cucumber fruit. All PG. cellulase and β-glucosidase activity may directly influenced by gene expression of these enzymes. Chitinase activity increased during fruit maturation,β-1,3-glucanase only had high activity in Mature fruit, and the gene expression level of both enzymes increased during growth. The water soluble pectin and protopectin content showed little change during the development and senescence of cucumber fruit, except Mature fruit had significant high level of water soluble pectin.PG and pectinase activity declined during cold storage and also increased before the end of cold storage, pectinase activity consistently increased during cold storage. PG activity decreased and pectinase activity slightly increased after rewarm. During cold storage cellulase and β-glucosidase activity slightly decreased in Immature and Yellow fruit, while in Mature and Breaker fruit both declined and then increased till end of cold storage. Fruit at earlier developmental stage have higher cellulase and β-glucosidase activity under chilling stress. Chitinase activity increased in all fruit under chilling stress, and higher chitinase activity appeared in more mature fruit which suffered less chilling injury. Change of chitinase activity inYellow fruit was different with other fruit, which showed declined trend. Under chilling stress,β-1,3glucanase activity increased in Immature and Mature fruit, but declined in Breaker and Yellow fruit, fruit at earlier developmental stage had higher β-1,3-glucanase activity than more mature fruit after rewarm. The water soluble pectin content increased, but protopectin content decreased in all fruit under chilling stress, protopectin content also increased after rewarm.