关键词: 2, 4-D, 沃柑, 氯氟吡氧乙酸异辛酯, 留树保鲜, ADH酶, PDC酶, 乙醇, 落果

Abstract:

【Objective】This study investigated the effects of fluroxypyr-meptyl treatment on fruit quality of Wuming Orah mandarin during on-tree preservation, aiming to provide a theoretical basis for finding alternatives to the commonly used 2, 4-D in citrus preservation, with a view to prolonging the market supply period of late-ripening citrus, and providing the technological support for high-quality development of citrus.【Method】Three treatments were applied to the stage of color change of Wuming Orah mandarin in Guangxi: water (CK), fluroxypyr-meptyl (YXZ) and 2, 4-D, with a total of three sprays at 15 d intervals. The first sampling was carried out at 20 d intervals after the last spraying, and then samples were collected every 15 days from mid-December to late March and divided into eight timepoints. The study measured the fruit's fruit drop rate and external quality, such as soluble solids, titratable acids, juice yield rate, ethanol, pyruvic acid and other physiological and biochemical fruit qualities. Additionally, metabolomics and RNA-seq analyses were also conducted, and the differential genes were verified by real-time fluorescence quantitative PCR (RT-qPCR).【Result】The fruit quality results indicated that YXZ treatment exhibited the similar effect on fruit with that of 2, 4-D. Compared with CK, the YXZ treatment group decreased the fruit drop rate by 0.81-9.76 percentage points, and increased the juice yield by 0.65%-3.56%, which could effectively maintain the internal quality of the fruit. During the on-tree preservation period, the content of ethanol, as the major off-flavor substance, was significantly decreased in YXZ treated fruits. Meanwhile, the content of pyruvic acid and the activities of ADH and PDC enzymes related to ethanol metabolism also decreased significantly. Additionally, the levels of stress-responsive primary metabolites such as proline and GABA also decreased, although the fruit quality metabolites were changed slightly. Weighted Gene Co-expression Network Analysis (WGCNA) identified nine co-expression modules, with the key module MEgreen showing significant positive correlations with ethanol levels and fruit drop (P<0.05). KEGG enrichment analysis revealed that it was mainly enriched in pathways related to glycolysis, hormone synthesis, and phenylpropanoid synthesis pathways, highlighting ethanol synthesis genes and jasmonic acid ZIM structural domain proteins from the MEgreen module. These genes’ expressions were inhibited by YXZ and 2, 4-D treatments, which was confirmed by qRT-PCR.【Conclusion】YXZ treatment effectively inhibited the expression of ADH, PDC and JAZ and reduced activity of ADH and PDC enzymes. Moreover, the treatment could decrease the ethanol and methanol content, lower the fruit drop rate, delay the reduction in juice yield, and maintain the quality of Wuming Orah mandarin while it reduced the activity of stress-responsive primary metabolites like proline. In sum, YXZ could serve as an alternative to 2, 4-D, extending the freshness period of Orah mandarin in Wuming area of Guangxi by approximately 30 days, allowing the latest harvesting period to be postponed until mid-March at the latest.

Key words: 2, 4-D, Orah Mandarin, Fluroxypyr-Meptyl, on-tree preservation, ADH enzyme, PDC enzyme, ethanol, fruit drop