Changes in Quality and Volatile Compounds of ‘Ready-To-Eat’ Kiwifruit with Transcriptome Insights

The developmental processes leading to the “ready to eat” ripening stage in kiwifruit differs by variety. In the present study, the effects of exogenous ethylene on quality and volatile compounds, as well as ripening-related gene expression, of red-fleshed kiwifruit cultivars (Actinidia chinensis cv. “Hongyang” and “Qihong”) were investigated. For postharvest ripening, uniform-sized fruits free from physical defects were treated with exogenous ethylene at 100 μL L−1 for 3 days at 25°C. At the “ready to eat” stage, soluble solids content was increased; however, firmness and titratable acidity decreased clearly in both cultivars. A total of 28 volatile compounds were identified including, 10 aldehydes & amines, 7 alcohols, and 11 esters. The ethanol and methyl butanoate showed strong associations with the ethylene treatment in both cultivars. In addition, results indicated that the “ready to eat” stage in red-fleshed kiwifruit could be distinguished by hexanal in “Hongyang,” and by both hexanal and (E)-2-hexenal in “Qihong.” Transcriptome analysis identified 121 and 1813 unigenes that were differentially expressed in the control 3-day and ethylene 3-day treatments vs. at harvest, respectively, in “Hongyang.” In “Qihong,” a total of 45 and 1136 unigenes were differentially expressed in the same comparison. Among these, genes involved in ethylene biosynthesis, cell wall degradation, and volatile emission, such as ACO, β-gal, PL, and Alcohol dehydrogenase, were significantly upregulated by ethylene treatment, which contributes to “ready to eat” quality of both kiwifruit cultivars.

猕猴桃‘即食’成熟阶段的发育进程因品种而异。本研究探究了外源性乙烯（exogenous ethylene）对红肉猕猴桃品种中华猕猴桃（Actinidia chinensis）‘红阳’与‘齐红’的品质、挥发性成分（volatile compounds）以及成熟相关基因表达的影响。针对采后成熟环节，选取大小均一、无物理损伤的果实，于25℃下以100 μL L⁻¹浓度的外源性乙烯处理3天。在达到‘即食’成熟阶段时，两个供试品种的可溶性固形物含量均有所提升，但硬度与可滴定酸度均显著下降。本研究共鉴定出28种挥发性成分，涵盖10种醛类与胺类、7种醇类以及11种酯类。乙醇与丁酸甲酯（methyl butanoate）在两个品种中均与乙烯处理呈现强相关性。此外，研究结果显示，红肉猕猴桃的‘即食’阶段可通过己醛（hexanal）鉴别‘红阳’品种，而‘齐红’品种则可通过己醛与（E）-2-己烯醛（(E)-2-hexenal）进行区分。转录组（transcriptome）分析表明，相较于收获时的样本，‘红阳’品种在对照处理3天与乙烯处理3天两组中分别鉴定出121个与1813个差异表达单基因（unigenes）；‘齐红’品种在相同对照比较下，分别鉴定出45个与1136个差异表达单基因。其中，参与乙烯生物合成、细胞壁降解与挥发性成分释放的基因，如ACO、β-半乳糖苷酶（β-gal）、果胶裂解酶（PL）与乙醇脱氢酶（Alcohol dehydrogenase），均经乙烯处理后显著上调，这有助于提升两个猕猴桃品种的‘即食’品质。