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.