Effects of rain-shelter cultivation on green leaf volatiles of ‘Cabernet Sauvignon’ grape

Abstract:【Objective】Volatile compounds are of importance to the quality of grape berries and wines.They largely determine the aromatic quality and variety characteristics. Green leaf volatiles (GLVs) areshort-chain acylic aldehydes, alcohols and esters produced by plants via enzymatic metabolism of polyun⁃saturated fatty acids. These compounds, which are normally the most abundant volatiles in grape berries,not only endow grape berry and wine with‘leaves’,‘grass’or‘fresh vegetative’odor, but also providethe direct precursors for the synthesis of hexyl acetate. Hexyl acetate in wines is found to be grape depen⁃dent and contribute to fruity character. Different from free-form volatiles, glycosidically bound GLVs ingrape berry are rarely paid attention on possibly because they don’t have direct contribution to the aromaattributors. But they possess potential importance in grape varieties used for making wine. In the wineproducingregions of east China, the quality of berries and wines are largely interfered with excessive rain⁃fall during grape maturation season. To minimize the declining of berry aroma quality, rain-shelter mea⁃surement has been implemented. The present study aimed at evaluating the effects of rain shelter on theaccumulation of GLVs and the lipid-derived esters in developing‘Cabernet Sauvignon’grape berries.The research outcome will provide theoretical basis for this cultivation mode application.【Methods】Theexperiment was carried out in a‘Cabernet Sauvignon’(Vitis vinifera L.) vineyard of the Chateau ChangyuAfip Global (44°30′N, 116°80′E), located in Miyun County of Beijing, in 2012 and 2013 vintages. Theown-rooted grapevines were planted in south-north row orientation with 1.0 m × 2.5 m spacing, and theywere trained into a sloping trunk with a horizontal cordon. The rain-shelter roof was set up on the 14thday after flowering (DAF, 14th June) in 2012 and 36 DAF (13th July) in 2013. A randomized block designwas adopted, and a total of six blocks were chosen to create three rain-shelter cultivation replicates andthree open-field cultivation replicates. Each block comprised 40 grapevines. To understand the change inmicroclimate around the grapevines under the rain-shelter and in the open field, dataloggers and probeswere installed around the clusters for collecting microclimatic data. Grapes were sampled at 20, 37, 50,64, 71, 78, 87, 97, 112, 125 and 135 DAF for the 2012 vintage and at 14, 28, 41, 57, 67, 78, 97, 112 and124 DAF for the 2013 vintage. The harvested samples were placed in plastic bags and quickly transportedto the laboratory. About 100 fresh berries per replicate were used for the determination of total soluble sol⁃ids and titratable acids. The remaining samples were immediately frozen in liquid nitrogen and stored at -80°C. Free-form volatile compounds were extracted from fruits by using headspace solid phase micro-ex⁃traction (HS-SPME) method, and glycosidically bound volatiles were separated through a Cleanert PEPSPEcolumn, followed by the hydrolysis by glycosidase AR2000 and the adsorption of HS-SPME. Thequalitative and quantitative analyses of free and glycosidically bound volatiles were performed via thetechnology of gas chromatography-mass spectrometry.【Results】Both total solar radiation and photosyn⁃thetically active radiation reaching the grape clusters were reduced by about 20%, and ultraviolet radiation was decreased by 40% under the rain shelter. With the growth and development of grapes, total soluble solids increased and titratable acids declined especially after veraison. The rain-shelter application,compared with the open-field cultivation, resulted in a significant increase in total soluble solid content inthe 2012 vintage, but no significant difference in both total soluble solids and titratable acids in the 2013vintage. Combined the two-year data, free-form straight-chain aliphatic alcohols, including (E )-2-hexenal and (E )-hexenoic acid with green leaf odor, under the open-field cultivation was higher than those under the rain- shelter cultivation. However, rain- shelter application raised the levels of glycosidicallybound straight-chain aliphatic alcohols, such as (E )-2-hexen-1-ol, hexanol and (E )-3-hexen-1-ol, aswell as free and glycosidically bound hexenal and hexenoic acid. Additionally, free and bound straightchainaliphatic esters (fruity aroma) were increased in the rain-shelter grape, including hexyl acetate, eth⁃yl octanoate, and (Z )-3-hexen-1-ol acetate. The two-way analysis of variance indicated that 22 freeformand 15 glycosidically bound-form volatiles dramatically differed between the vintages, and 6 freeformand 4 bound-form GLV compounds varied with the cultivation treatments. F test showed that vintageinfluence was stronger than the cultivation mode. There was significant interactive effect of vintage andrain-shelter treatment on the concentrations of 6 free-form GLV compounds including (E)-2-hexenal,nonanal, decanal, 1-hexanol, (E)-2-hexenol and 1-octanol. In addition, four bound-form GLV com⁃pounds, decanal, 1-hexanol, 1-heptanol and 1-dodecanol, also exhibited a significant difference. Therewas difference in the total rainfall (524.2 mm and 465.6 mm) existed in the whole period of berry development between 2012 and 2013 vintages. Excessive rainfall was considered to interfere with the accumulation of green leaf volatiles, but the rain-shelter application could effectively avoid the effect, which wasdisplayed in the 2012 vintage with more rainfall compared to the 2013 vintage.【Conclusion】Comparedwith the open-field cultivation, rain-shelter cultivation in this study greatly lessened the green leaf odorof grape berries and consequently improve fruity aroma of grapes and wines. The improvement in total soluble solids and fruity aroma under the rain-shelter was significant in the vintage that received higher rainfall. Based on our results, it is concluded that simple rain-shelter cultivation can effectively relieve the deterioration of berry quality especially in the vintage with much rainfall.