Advances in postharvest biology and regulation techniques for prevention of fruit quality deterioration in peach

Abstract: The total area of peach cultivation in our country is 100 hectares, and more than 80% are produced for fresh sales. Peaches are easy to deteriorate at room temperature, long-term cold storage can result in internal browning (IB), loss of flavor, reduction of disease resistance and accumulation of harmful substances, which are all the key postharvest problems in peach industry. With the upgrading of varieties and the flesh texture diversification of peach [melting (MF), non-melting flesh (NMF), stony hard (SH), and slow ripening (SR)], postharvest quality deterioration symptoms as well as related metabolic enzymes, proteins and genes show different responses to fruit senescence and regulation techniques. To understand the research situation, the author has summarized and analyzed the research progress of postharvest quality deterioration biology, regulation technology, industrial application of fresh-keeping products and technologies, and also put forward the shortcomings and development trends. At present, the biology of postharvest quality deterioration is mainly limited to the excavation and verification in functional proteins and genes of fruit texture, internal quality and chilling injury (CI). Fruit softening is a complex process, including cell wall degradation, ethylene metabolism and other metabolic changes. Among them, the gene PpPG is a biomarker of fruit softening and cell wall degradation, and ethylene is the direct factor that leads to fruit softening. Ethylene response factors like PpERF/ABR1 and PpERF61can also regulate ethylene biosynthesis and fruit softening by activating the promoter of PpPG genes and ripening-related genes. Sugar loss, energy deficiency, active oxygen accumulation, abnormal metabolism of endogenous hormones and gene methylation are the key factors leading to CI by affecting membrane system and ROS. Five structural genes (PpSS, PpINV, PpMGAM, PpFRK and PpHXK) and eight transcription factors (PpMYB1/3, PpMYB- related 1, PpWRKY4, PpBZIP 1/2/3 and PpbHLH2) jointly regulate the sugar metabolism and cold resistance. Down- regulating the expression of PpVIN2 can improve the sucrose content and inhibit CI, and PpeSOT3 may be a potential key gene affecting sorbitol metabolism and chilling resistance. Plant endogenous hormones such as ethylene, abscisic acid (ABA), β-aminobutyric acid (BABA) and salicylic acid (SA) also play an important role in regulating fruit senescence and CI. Postharvest disease is one of the key problems that cause post- harvest loss. Phomopsis sp., Botrytis cinerea, Colletotrichum siamense, Rhizopus sp., Fusarium sp. and Aspergillus sp. are the main pathogens causing postharvest rot. The decrease of lactone, ester and linalool contents and the accumulation of aldehyde and alcohol can be used as predictors of quality deterioration. The metabolism of lignin and aldehydes is the key metabolic pathway to regulate postharvest diseases. Based on the above background, the following suggestions are put forward: (1) Reveal the relationships among cell wall ultrastructure, softening markers, factors affecting ethylene metabolism, as well as interaction mechanism. (2) Expound the regulation mechanism of sugars, acids (pay more attention to the regulation mechanism of sugar metabolism and energy. Acid is also the important substrate of postharvest metabolism that makes great contributions to fruit flavor, energy metabolism and cold resistance, however, its metabolic mechanism is seldom studied.) and volatile substances (especially the accumulation mechanism of alcohols and aldehydes) on postharvest quality. (3) Determine the regulation of cold resistance by sugar and energy metabolism, antioxidant system, endogenous hormones (especially key genes and related factors of ABA metabolism) and key genes methylation. (4) Disclose the mechanism of lignin metabolism and the accumulation of alcohols and aldehydes in the prediction and regulation of postharvest diseases, which can provide a theoretical foundation for the development and application of regulation technology. Temperature, 1-MCP, UV, gas, exogenous hormone and biocontrol bacteria treatment have been applied in peach storage. Among them, storage temperature is the first factor affecting the fresh-keeping efficacy. For example, near- freezing temperature (NFT), low temperature conditioning (LTC), intermittent warming (IW), heat shock treatment (HST) and cold shock treatment (CST) can prolong storage period by inhibiting fruit softening and enhancing the cold tolerance. 1-MCP has a significant regulatory effect on fruit softening, flavor loss, postharvest diseases, especially on softening, by reducing the activities of PG, PME and PEL, down-regulating the expression of PpPG1, 2, PpPME1, 2 and PpPEL1, 2 under different storage conditions. Proper concentration of gas and exogenous hormones can improve the cold resistance. CO2 and NO treatment can reduce CI and improve the cold resistance by regulating cell wall and lipid metabolism, such as by regulating the expression of LOX, ADH, FAD and related genes, activating the antioxidant system, and maintaining higher energy charge. Exogenous hormone treatment can also significantly improve the cold resistance during cold storage, among which MeJA, SA, γ-aminobutyric acid (GABA), melatonin (MT) and jasmonic acid (JA) have better effects. Although the postharvest storage technologies have been widely studied, it has not solved the problems of fruit disease resistance reduction, flavor loss and shelf life shortening caused by long-term cold storage, and its application in industry still has some limitations. It is suggested that the industrialization application of technologies and products should start from the followings in the future: (1) The establishment of cold chain logistics system for peach harvest, which integrates classification, packag-ing, pre-cooling, storage, transportation and shelf-life technical parameter. (2) The research and development of compound antistaling agent and the matching technologies, as well as combination of new antistaling agent with post-harvest packaging materials to improve the application of technology and products in industry. (3) The research and development of terminal shelf-life technology and exogenous hormone dormancy breaking technology, in order to solve the problem that the fruit shelf cannot break dormancy normally at room temperature after long-term low-temperature storage. (4) The establishment of cold resistance and disease prevention system technology and the breeding of cold-resistant varieties, in order to prolong the supply period of high-quality fresh peaches and solve the technical bottleneck problem of“the Belt and Road”(15-30 days ocean transportation and terminal shelf technology) of domestic fresh peaches.