Comparative analysis of storage properties and pectin metabolism between Zhufeng and Huanghuayou papaya varieties

[Objective] The aim of this study was to investigate the physiological processes involved in post-harvest softening of papaya fruit and to understand the factors leading to differences in storage characteristics between two varieties. The results will serve as a reference for improving papaya preservation during storage and transportation, and for breeding high-quality and storage-resistant varieties. [Methods] Using Zhufeng and Huanghuayou papaya fruits as experimental materials, physiological indicators, such as color parameters (a*, hue angle, color index, and chroma), firmness, water loss rate, respiration rate, ethylene production, TSS and TA contents, were measured during post-harvest storage. The levels of protopectin, water-soluble pectin (WSP), covalent-soluble pectin (CSP), and ionic-soluble pectin (ISP) were analyzed, and the activities of pectin methylesterase (PME), polygalacturonase (PG) and pectin lyase (PL) were examined. Gene expression levels related to PMEs and PGs were also determined to identify the main factors contributing to the differences in storage characteristics between the two varieties. [Results] During storage, both papaya varieties showed color changes and fruit softening. In terms of appearance, the hue angle of both varieties decreased, while the a* value and color index gradually increased, with the skin color changing from green to yellow. At later stages of storage, Zhufeng showed an orange-red color, while Huanghuayou showed an orange-yellow color. In terms of storage performance, the firmness of both Zhufeng and Huanghuayou papaya fruits gradually declined during storage. Huanghuayou exhibited rapid softening, with firmness dropping sharply to below 10 N within 0-7 days after harvest (DAH), before slowing down in the later storage phase. In contrast, Zhufeng demonstrated superior storage resistance, maintaining higher firmness and only decreasing below 10 N after 11 DAH. Throughout the entire storage period, Zhufeng consistently retained greater firmness compared to Huanghuayou. As storage progressed, respiration rates increased for both varieties, but the rate for Zhufeng remained significantly lower than that of Huanghuayou. Water loss analysis showed that during the initial storage period, Huanghuayou had a higher rate of water loss compared to Zhufeng, whereas at the later stages, Zhufeng exhibited a higher water loss rate than Huanghuayou. Additionally, ethylene release followed a peak-like pattern in both varieties, with Zhufeng reaching its ethylene peak at 7 DAH, three days later than Huanghuayou. These results suggested that Zhufeng displayed slower softening, a lower respiration rate, a delayed ethylene peak, and overall better storage characteristics. After 11 DAH, Zhufeng preserved higher TSS content and lower TA levels, maintaining its commercial value and offering a shelf life of up to 11 days under normal conditions. Significant differences in pectin components and related enzyme activities were observed between the two cultivars. During storage, protopectin contents in both Zhufeng and Huanghuayou papaya fruits steadily decreased, while the concentrations of WSP, CSP and ISP consistently rose. This suggested that protopectin hydrolysis occurred, generating WSP, CSP, and ISP, and caused gradual degradation of cell structure. Zhufeng fruit showed significantly higher protopectin content compared to Huanghuayou, while WSP and CSP contents were significantly lower; ISP content showed no significant difference. It suggested that the protopectin in Zhufeng papaya underwent minimal depolymerization during storage, allowing the fruit to retain its remarkable firmness, whereas ISP levels may not directly correlate with fruit firmness variations. During the storage period of Zhufeng and Huanghuayou papaya fruits, PME activity showed a steady upward trend. At 7-11 DAH, PME activity in Zhufeng fruits was notably lower compared to Huanghuayou. Polygalacturonase (PG) activity exhibited more pronounced changes, with a continuous increase throughout storage, while Zhufeng consistently demonstrated lower PG activity than Huanghuayou. These findings highlighted the critical roles of PME and PG in the softening process, with their activity levels possibly driving the distinct softening traits of the two varieties. However, PL activity showed no significant difference between the Zhufeng and Huanghuayou, indicating that it might not be responsible for the differences in softening. RT-PCR was used to investigate changes in the expression levels of six PG genes (CpPG1, CpPG2, CpPG3, CpPG4, CpPG5 and CpPG6) and five PME genes (CpPME1, CpPME2, CpPME3, CpPME4 and CpPME5) during the storage of Zhufeng and Huanghuayou papaya fruits. The analysis revealed that while CpPG3, CpPG4, and CpPME1 showed no detectable expression, the remaining genes displayed significantly different expression patterns between the two varieties. Gene expression analysis suggested reduced levels of CpPG2, CpPG5, CpPME2, CpPME4, and CpPME5 in Zhufeng compared to Huanghuayou. Additionally, CpPG1 and CpPG6 peaked later in Zhufeng, while CpPME3 was expressed at higher levels in Zhufeng than in Huanghuayou. This suggested that CpPME3 may play a key role in controlling the softening process in Zhufeng papayas. [Conclusion] Variations in protopectin, WSP and CSP levels, and PME and PG activities are the key factors contributing to the distinct storage characteristics of Zhufeng and Huanghuayou. Genes such as CpPG1, CpPG2, CpPG5, CpPG6, CpPME2, CpPME3, and CpPME4 may play a role in the post-harvest softening process across papaya varieties with different storage resistance.

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