Advances in the effects of preharvest treatments on fresh-keeping of postharvest fruits

Abstract: The life activities of the postharvest fruits determine the commercial quality of the fruits, and the preharvest treatments are important for the life activities, quality and shelf-life of the postharvest fruits. Researchers have paid more attentions on the importance of preharvest application of plant regulators for enhancing the quality and storability of postharvest fruits for several decades. Thus, lots of work focusing on the postharvest effectiveness of preharvest treatments have been carried out constantly. This paper reviews the domestic and abroad efforts on developing preharvest treating strategy and investigating their effects on fresh- keeping and physiological activities. Generally, the preharvest treatment refers to the handlings of spraying regulator solutions three to five times on the leaves or fruits at full-bloom stage, young fruit stage, rapid growth stage and/or fruit ripening stage. The treating agents are usually naturally acquired chemicals eco-friendly and easy to absorb. The plant growth regulator is the largest category of the treating agents, which mainly includes gibberellin, diethyl aminoethyl hexanoate, salicylic acid, jasmonic acid and their precursors. Moreover, chitosan and calcium salts are widely and frequently used agents. Other chemical regulators like 1-methylcyclopropene and sodium diethyl dithiocarbamate are also employed in the studies. Preharvest spraying with these agents is capable of en-hancing and maintaining fruit quality at harvest and during postharvest storage: the accumulation of pigments like anthocyanin could be elevated before harvest, and the postharvest de-greening or coloring might be postponed via regulating ripeness and senescence; the fruit firmness is closely associated with cell wall polysaccharides like pectin, cellulose, and hemi-cellulose, which could be retained after preharvest handlings via suppressing the degrading enzymes like polygalacturonic acid, pectin methylesterase, cellulase and corresponding genes; the flavor characteristic largely depends on the carbohydrates, organic acid and volatiles, and their accumulation can be improved by preharvest treatments to retard postharvest flavor decay. These effects of preharvest treatment on quality retention are reflections of its postharvest physiological impacts, which mainly include the inhibitions on the respiration, ethylene synthesis and action, water loss, and metabolic disorder, and the improvements in the refrigeration adaptability. As the key metabolism of postharvest fruit, respiration signifies the intensity of ripeness and senescence progress in relation with the storability and shelf-life, while the endogenous ethylene is well acknowledged for dominating the respiration and accelerating fruit aging. Application of preharvest regulator could inhibit the respiration and ethylene production via influencing the physiological status during growth and at harvest, or blocking the gas exchange between fruits and environment, and further suppressing ethylene synthesis. Postharvest fruit water loss due to respiration and transpiration is an important accelerator of metabolic dysfunction. Some agents used in postharvest water retention handling like chitosan are also employed in preharvest treatment and exhibited sound effects, but its function would not be the same as postharvest coating and would deserve more investigation. Another key factor of physiological dysfunction is the loss of cellular structural and functional integrity, which is closely associated with the attack of reactive oxygen species (ROS) and energy deficit. Preharvest spraying with regulators might induce higher oxidation resistance to delay the production and accumulation of ROS via improving levels of antioxidants and ROS-scavenging enzymes. It has been reported that the energy status during storage could be maintained after preharvest treatment, protecting the cellular structure and retarding the metabolic disorder. The cold chain has become a common measurement for postharvest handling, distribution, and marketing, while chilling injury is hard to avoid during these processes for cold sensitive fruits. Many researches found that preharvest treatment with plant growth regulators like gibberellin, salicylic acid, and jasmonic acid has notable effect of inducing the cold resistance of the cold sensitive fruit like peach, plum and pineapple. The main inducing mechanism has been reported to be related to higher ROS- scavenging system protecting the compartmentalization in cell and reducing the membrane lipid peroxidation. On the other hand, preharvest handlings may also prolong fruit storage period via inhibiting postharvest diseases. Since most fruit pathogenic invasions are latent infection, preharvest prevention is a critical measure. Most preharvest regulators such as salicylic acid, jasmonic acid, gibberellin, and chitosan are natural antimicrobial agents capable of preventing spoilage organisms. More importantly, they are also disease resistance inducers that has long-term enhancement on the resistant proteins such as phenylalnine ammonialyase, β- 1,3- glucanase and peroxidase, and increases the disease resistant compounds like phenolics. Therefore, preharvest application of regulators is a highly feasible and effective handling to improve fruit commercial quality, storage adaptability and resistance to diseases, and its mechanism of action is an integrated function of physiological impacts that still demands lots of constant investigation. This paper would provide an important reference for the research, extension, and development of fresh-keeping oriented preharvest handling technology.