Preharvest GA3 sprays combined with postharvest hot water dipping control of peel browning in‘Pingguoli’pear fruit during low temperature storage and its partial mechanism

Abstract:【Objective】‘Pingguoli’is a famous pear cultivar in China, however, peel browning of the fruitis a major physiological disorder during postharvest, which results in many different size dark brown spotson the peel, significantly damaging the appearance quality of the fruit. Therefore, it is necessary for thepear industry to find an effective strategy to control peel browning of‘Pingguoli’pear fruit. The aim ofthis study is to investigate the effects of spraying gibberellin three times during fruit development and post⁃harvest hot water dipping on the peel browning of‘Pingguoli’pears during low temperature storage, andto explore its partial mechanism of these treatments.【Methods】The 17 year old pear trees were selectedand sprayed three times with gibberellin at 50 mg·L^-1 during fruit development, during the young fruit pe⁃riod, fruit enlarging period and maturation period, with water sprayed as the control. We selected the com⁃mercial mature and uniform size fruit, without disease and pests, for our experimental materials. Harvested pears were dipped in hot water at 45 ℃ for 15 min. The treated fruits were stored at low temperature (0±1 ℃) to investigate the effects of preharvest gibberellin spray combined with postharvest hot water dippingon the index of peel browning of‘Pingguoli’pears during storage by using a statistical method. The spec⁃trophotometric method was used to analyze the content of the phenolics, integrity of the cell membrane,the activity of superoxide dismutase, catalase and peroxidase. A scanning electron microscopy and transmission electron microscopy were employed to observed the differences between the control and the treated pears on the fruit surface structure and ultrastructure of epithelial cells. Microsoft Excel 2007 softwarewas used to calculate the mean and standard deviation and Origin 8.0 was used to make graphics.【Re⁃sults】Both gibberellin and hot water treatments significantly decreased the index of peel browning of fruitduring low temperature storage. However, the combined treatment showed the best results, followed by thegibberellin treatment. The index of peel browning of the combined and gibberellin treatments was only30.8% and 46.1% of the control after 120 d of storage. Gibberellin and hot water treatments inhibited ac⁃cumulation of phenolic, reduced the activity of polyphenol oxidase; the combined treatment showed thebest results, followed by the gibberellin treatment. Compared with the control, the combined treatments re⁃duced 10.6% of the contents of phenolic after 60 d of storage, and decreased 34.2% of polyphenol oxidaseactivity after 30 d of storage. Gibberellin and hot water treatments maintained the integrity of cell mem⁃brane, the combined treatment showed the best results. The cell membrane relative leakage rate of thecombined treatment reduced 23.82% of the control after 90 d of storage, the content of malonaldehyde wasonly 50% of the control after 60 d of storage. The treatments enhanced activities of superoxide dismutase,catalase and peroxidase, the combined treatment showed the best results, the activity of superoxide dis⁃mutase, catalase and peroxidase were 1.0, 0.77 and 0.64 folds higher than the control after 90 d of storage,respectively. Observations of scanning by an electron microscope indicated the peel of the control wassmoother, had a rough structure and was fully covered with uneven mesh cracks around the crack wax.However, after gibberellin combined with hot water treatment, the fruit epidermal wax layer became thick⁃ened, had an uneven distribution and showed a layered structure with the edges of the layered structuresignificantly raised, and the peel had a few cracks. Observations with a transmission electron microscopeshowed that the peel wax of the control was thinner, the cuticle was not complete, the cell wall was col⁃lapsed and the layer had disappeared. However, the peel wax of the combined treatment was significantlythickened, the boundaries of the horny and wax layer were visible and the cell wall had structural integri⁃ty. Moreover, the electron density of mesoglea was higher and the primary wall appressed the two sideswhich maintained the integrity of the cell structure.【Conclusion】Preharvest gibberellin spraying duringfruit development combined with postharvest hot water dipping shows the best control of peel browning of‘Pingguoli’pears during low temperature storage. The controlling mechanism is closely related to maintain membrane integrity, increase activities of antioxidant enzymes, decrease the accumulation of phenolicand increase the thickness of the surface protection tissue. Therefore, preharvest gibberellin sprayed threetimes during fruit development combined with postharvest hot water dipping is an effective measure to con⁃trol peel browning of‘Pingguoli’pears during low temperature storage.