Effects of shading on the aroma component of fruits at different developmental periods in Hongyang kiwifruit

Abstract: 【Objective】Aroma is an important trait of fruit quality, constituted by a variety of compounds including esters, aldehydes, ketones, alcohols, terpenes and volatile phenols, as well as a rangeof other aromatics. The compound and content of aroma undergo consistent changes throughout fruit development, and can be affected by various environmental factors. It has been demonstrated that sunlight, as a crucial factor influencing fruit development, has a significant effect on the content and composition of aroma. Bagging is widely practiced in fruit cultivation. The fruit with bagging tends to exhibit a superior external appearance and neater shapes compared to non-bagged, which makes them more popular by consumers. However, the practice of bagging limits the fruit’s exposure to sunlight, which in turn affects the formation of fruit aroma. In this study, the effects of shading on fruit aroma types and content at different developmental periods were determined to explore the significance of each period in aroma compounds synthesis, and identify the optimal timing for bagging. Furthermore, a transcriptome analysis was employed to explore the molecular mechanisms underlying the impact of shading on aroma synthesis. The findings of this study will serve as a valuable reference for the refined cultivation of kiwifruit.【Methods】The experimental design spanned from 20 days after anthesis (20 DAA) to 140 DAA, dividing the maturation process into six periods (periods Ⅰ to Ⅵ) with 20-day intervals. Each period underwent light exclusion using double-layer black bags (Groups T1 to T6), with T7 representing total shading and CK representing no shading. The fruits were harvested when the soluble solids content reached 7% and allowed to ripen at room temperature until the soluble solids content reached 20%. Aroma compounds were then analyzed by headspace solid-phase microextraction (HS-SPME) and gas chromatography-mass spectrometry (GC-MS). Data were processed and analyzed using Excel 2016 and SPSS 20.0, and graphs were created using Origin 2021. Samples from T7 and CK fruits were sent for transcriptome sequencing and KEGG and GO enrichment analysis of DEGs were conducted using the company's platform.【Results】A total of 38 aroma compounds were detected in Hongyang kiwifruits with different treatments, including 15 esters, 11 aldehydes, 8 terpenes, 3 alcohols and a single other compound. The number of aroma compounds in each group ranged from 19 to 37, with T6 exhibiting the highest diversity at 37 types. In contrast, T2 and T5 exhibited the lowest diversity, with both groups containing 19 compounds. The highest total aroma content was observed in T4, followed by T2 and T5 among the groups with shading. The proportions of the major aroma constituents in the Hongyang kiwifruit remained largely unchanged by the exclusion of sunlight. Ester compounds consistently accounted for over 90% of the total content in each treatment group, with aldehydes comprising 1% to 7% and terpenes and alcohols each accounting for less than 2%. The ester content was found to be significantly reduced under shading in all groups with the exception of T4, which exhibited the highest ester content, followed by T2 and T5. The aldehyde content of T4, T5 and T6 exhibited no significant difference compared to CK, whereas the other groups displayed a significantly lower content than CK. The terpene levels in T1 and T2 were not significantly different from CK, which exhibited no accumulation in the fruit of T4. In contrast, the other treatment groups displayed significantly higher terpene levels than CK. Furthermore, all groups except T4, which was bagged, showed an increase in terpene compound diversity, with the highest variety found in T7, followed by T6. The only treatment group in that the alcohol content was significantly higher than CK was T6. No accumulation of alcohol was observed in T2 and T5, while all other treatment groups exhibited lower alcohol levels compared to CK. The results of the cluster analysis indicated that the aroma composition of T4 was most similar to that of CK. T2 and T5 were grouped together, while T1 and T3 were placed in a separate cluster. T6 and T7 were classified into their own distinct categories. The principal component analysis demonstrated that T6 exhibited the highest overall score, followed by T4, T2 and T5. Transcriptome analysis identified 1, 248 differentially expressed genes, with 788 downregulated and 460 upregulated ones in T7 compared to CK. 15 genes relat-ed to aroma synthesis were selected, with shading resulting in reduced expression of ethanol dehydrogenases (ADH), alanine aminotransferase (AGXT), and aspartate aminotransferase (GOT) genes in T7 fruit, while increasing the expression of pyrophosphate decarboxylase (MDV), and aldehyde dehydrogenases (ALDH).【Conclusion】The impact of shading in different developmental periods resulted in varying degrees of aroma content and compound constitution, indicating that the synthesis of various aroma components in Hongyang kiwifruit occurred at disparate optimal periods. The results demonstrated that period Ⅵ was a critical period for ester synthesis; periods Ⅰ to Ⅲ were significant for the accumulation of aldehydes; and period Ⅳ was crucial for terpene synthesis. Comprehensive analysis showed that kiwifruits from T4, T5 and T2 had higher aroma content and demonstrated reduced sensitivity to shading, showing superior aroma quality. Shading suppressed the expression of ADH, AGXT and COT1 genes while increased the expression levels of MVD and ALDH genes, which may account for the increased content of terpene aroma compounds and the decreased levels of ester, aldehyde and alcohol compounds in shading fruit. Thus, period Ⅱ (40-60 DAA) and periods Ⅳ to Ⅴ (80-120 DAA) were identified as suitable periods for bagging. Combined with previous research, it was recommended to begin bagging kiwifruits in the late portion of period Ⅰ and continue from period Ⅱ to period Ⅴ (40-120 DAA). The bagging process should be discontinued upon the completion of period Ⅴ to permit the fruits to receive sunlight in a natural manner. These results can serve as a valuable reference for the precise management of kiwifruit cultivation.