Effects of bagging treatments with different materials on aroma components and their biosynthetic gene expression in ‘Shine Muscat’ grape berry

Abstract:【Objective】Aroma is one of the important sensory indexes related to fruit flavor. The flavor ofgrape berry consists of sweetness, sourness and aroma. Bagging can improve the appearance and internalquality of the fruit. Yet its effect may vary greatly among different bagging materials, which regenerate different microclimates. Temperature and humidity affect enzyme activities and respiration rate, and light intensity affects the supply of assimilates. In this study, experiments were performed to investigate the effectsof fruit bagging with 4 types of bags on fruit flavor quality and genes expression in‘Shine Muscat’grape inorder to understand the regulation mechanisms of aroma biosynthesis and the impact on the expression ofkey enzyme genes in monoterpene biosynthesis pathway, and to select the appropriate fruit bagging materials for commercial production. The 4 types of bags were the ordinary monolayer white paper bags (whitebags for short), transparent micro-porous plastic bags (transparent bags for short), yellow outside black in⁃side double-layer paper bags (black bags for short), and monolayer green paper bags (green bags forshort), and bagging with white bags was used as the control.【Methods】The samples were collected every10 days from July 25 for 6 times. The samples were collected randomly from both the shade and sun sideswith 3 biological replicates each with about 50 -60 berries. The expression of genes including DXS3,GPPS and LIS were analyzed using qRT-PCR. Berry diameters and weight were collected using an elec⁃tronic vernier caliper and an electronic balance accuracy (0.01 g), respectively. A handheld sugar meterwas used to determine the Brix of the berries bagged with the 4 types of bags, and titration method used todetermine the titratable acid content. The samples collected on September 5 were used to detect the aromacomponents with GC-MS. Juice was extracted after cleaning and centrifuged. 8 mL of the clear supernatant was transferred into a 15 mL bottle, to which 3.0 g NaCl and 5 μL 3-octanol (818 mg·L^-1) were add⁃ed as the internal standards. The bottles were sealed, water bathed at 50 ℃ for 30 min, inserted with a solid-phase micro-extraction head and magnetic stirred at 950 r·min-1 and 50 ℃ for 40 min. After adsorption, the sample was transferred to the gas chromatographic injector set at 220 ℃ to analyze for 2 min.【Results】The quality parameters of fruit included size, weight, sugar-acid ratio and so on. Comparedwith the control, fruit bagged with transparent bags had a lower content of titrable acidity and a higher content of soluble solids, and therefore their flavor was better. The content of soluble solids in fruit treatedwith green bags and black bags was lower, and hence their fruit flavor was lighter. DXS3, GPPS and LISgenes were the key genes in monoterpene synthetic pathway. The expression levels of these genes affectmonoterpene synthesis, thus affecting the rose flavor. The expression of DXS3, GPPS and LIS were up-regulated with transparent bags, which was beneficial to monoterpene generation. The peaks of DXS3, GPPSand LIS expression were delayed and down-regulated by black bags, which was negative to monoterpeneproduction. Green bags had no significant effect. Fruit bagging significantly influenced the aromatic composition. 37 species of aromatic components were detected in white bag treatment, and 41 were found infruit treated with transparent bags, 4 more than the control. The number of aromatic components was only 25 in fruit treated with green bags, while it was 33 in black bag treatment. The aromatic components included alcohols, aldehydes, ketones, esters, alkenes, alkanes, phenolic compounds, and others. The relative contents of alcohols and aldehydes were highest and determined the overall fragrance in‘Shine Muscat’grapes. The relative contents of ketones, esters and others were lower. Transparent bag treatment increased the species of aromatic components and relative contents of terpenoids such as linalool, cis-linaloloxide and geraniol, and strengthened the flavor. The relative contents of terpenoids in fruit bagged withblack bags were reduced. The species of aromatic components in fruit bagged with green bags were thefewest.【Conclusion】Fruit bagging with different bags had significant difference on aroma componentsand the expression of key biosynthesis genes in‘Shine Muscat’grape.