Effects of modified atmosphere treatments on chilling injury and flavor quality of peach fruit during storage

Abstract：【Objective】Peach fruit is perishable and easy to deteriorate at room temperature, and cold storage is an effective way to prolong storage life. Peach fruit is chilling-sensitive, and the loss of fruit quality after postharvest cold storage caused by chilling injury (CI) reduces consumer satisfaction. Modified atmosphere (MA) has been extensively used to alleviate chilling injury (CI) of horticultural produces. However, in some cases, the flavor quality decreases due to excessive deficiency in oxygen and accumulation of carbon dioxide in MA containers. In peach, the effects of MA treatment on fruit flavor quality suffering from CI remain largely unknown. In this study, Hujingmilu and Zhonghuashoutao peach fruits were used as materials to explore the effects of MA treatments with different O2 concentrations, on chilling injury and flavor quality of peach fruit during storage. Meanwhile, through analyzingthe correlation between the gene expression of ADH and PDC, and accumulation of acetaldehyde and ethanol, the study aimed to identify the critical members involved. Finally, the study was carried out to determine the most suitable O2 concentration for MA storage of peach fruits.【Methods】The peach fruits were pre-cooled at 0 ℃ for 12 hours to dissipate the field heat, and then divided into four groups, with final O₂ concentrations controlled at 1%, 3%, 5% and non-control for MA1, MA2, MA3 and control, respectively. Hujingmilu and Zhonghuashoutao fruits were stored at 0 ℃ for 40 d and 60 d, and then put on 20 ℃ shelf for 3 d and 4 d, respectively. Firmness, ethylene production, internal browning (IB) index, contents of soluble sugars, organic acids, ethanol and acetaldehyde were measured. The ADH and PDC gene families were analyzed in silico, and the expression of main expressed members PpPDC1 and PpPDC2 as well as PpADH1, PpADH2 and PpADH3 was determined by RT-qPCR. The relationship between gene expression of PpPDCs and PpADHs, and contents of acetaldehyde and ethanol were analyzed.【Results】Hujingmilu and Zhonghuashoutao peach fruits suffered from serious IB on shelf following cold storage, with index as high as 0.53 and 0.69, respectively. All three MA treatments effectively inhibited the IB of both cultivars, with indexes all below 0.1. With Hujingmilu, the MA treatment also effectively alleviated the impairment of cold storage on fruit softening. With Zhonghuashoutao, the ethylene production of the fruits with MA1 and MA3 treatments was significantly higher than that of control, indicating that MA treatment guaranteed the normal ripening of the fruit. The ethanol and acetaldehyde contents, however, varied greatly among different treatments. Compared with control, ethanol and acetaldehyde significantly accumulated in fruits of MA1 and MA2, while there was no marked difference in fruits treated with MA3 or control. During the shelf life, the accumulation of ethanol in all treatments was reduced, while for acetaldehyde, no significant change was observed. A total of 5 PpPDCs and 33 PpADHs were identified from genome database of peach. PpPDC1 and PpPDC2, as well as PpADH1, PpADH2 and PpADH3 were identified as highly expressed members and hence used for further analysis. The expression patterns of PpPDCs and PpADHs in the postharvest storage of peach fruits were differential between fruits at cold storage and those on shelf. For example, the expression of PpADH1 during cold storage was significantly higher than that on shelf, with maximum difference being up to 17.19 times, while PpADH2 and PpADH3 showed opposite trends. The expression of PpPDCs during cold storage was generally higher than that on shelf. The expression patterns of PpPDCs and PpADHs were also quite different under MA treatments with different O2 concentrations. Overall, the expression of PpPDCs and PpADHs in fruits of MA1 was highest, while that in fruit of MA3 was lower. At the end of the cold storage period of Hujingmilu and Zhonghuashoutao (40 d and 60 d), the expression level of PpADH1 in fruit of MA3 was only 15.48% and 43.24% of MA1, and for PpPDC2, only 15.11% and 66.93% in two cultivars, respectively. Correlation analysis showed that the expression of PpPDC2 and PpADH1 were significantly and positively correlated with the accumulation of ethanol and acetaldehyde.【Conclusion】Compared with control, all MA treatments, with final O2 concentrations ranging from 1% to 5%, can effectively alleviate chilling injury. MA1 and MA2 treatments led to significant accumulation of ethanol and acetaldehyde, while MA3 did not. Among the 5 PpPDCs and 33 PpADHs, PpADH1 and PpPDC2 played important roles in accumulation of acetaldehyde and ethanol during postharvest storage of peach fruit. MA3 treatment inhibited the expression of PpPDCs and PpADHs, and prevented the excessive accumulation of ethanol and acetaldehyde. The MA treatment for peach was optimal with final O2 concentration controlled at 5%.