Effect of cluster thinning on sugar/acidity and the accumulation of isoprene-derivated volatiles in‘Cabernet Sauvignon'grape berries

Astract: 【Objective】Both terpenoids and C13-norisoprenoids are generated from the isoprene metabolic pathway. These volatiles generally possess very low sensory threshold and pleasant aroma attributes, thus making an important contribution to the floral and fruity odor of grape berry and wine. Cluster thinning is usually required in viticultural management. Previous studies have demonstrated that suitable level of cluster thinning helps to improve the flavor and aroma quality of grape berry and wine. By contrast, either too high or too low fruit loading will cause negative effects on the accumulation of the compounds having sensory importance. And the cluster thinning level depends upon grapevine growing environment to a large extent. Xinjiang is a unique region of grape and wine production in China, and the climate of this region is characterized by long sunshine time, strong solar intensity, little rainfall and big diurnal temperature difference. However, the effect of cluster thinning on the volatile accumulation under this particular climate has been poorly understood up to now. The present study aimed at understanding the effects of two cluster thinning treatments on the composition and concentrations of free-form and glycosidically bound isoprene volatiles in developing‘Cabernet Sauvignon' (Vitis vinifera L.) grape berries in Manasi, Xinjiang. The research outcome will provide valuable reference for the improvement of wine aroma qualityin this region, and lay a foundation for further study on isoprene metabolism and regulation in grape berry.【Methods】The cluster thinning treatments were conducted in the two vineyards belonging to the Xinjiang CITIC Guoan Wine Co. Ltd located in Manasi county, Xinjiang. The soils of the two vineyards, called as Guangdongdi and Yuanyichang, possessed different water holding capacity and organic matter content, and all the grapevines were trained into the system of modified vertical shoot position (M-VSP) with 12-15 shoots per vine. The grapevines were cluster-thinned at post-fruit set [Eichhorn-Lorenz (E-L 13 stage) ] to retain one cluster per shoot (CT1) and two clusters per shoot (CT2) . The randomized block design was adopted in this study, and each cluster thinning treatment in a vineyard was carried out in three blocks, which corresponded to three biological replicates. The grape berries were collected at the indicated stage during berry development. The actual cluster number per shoot, berry weight and berry yield per vine were examined at the ripening harvest. The free-form and glycosidically-bound isoprene volatiles in the grape berries were qualitatively and quantitatively analyzed using the head space-soild phase microextraction (HS-SPME) combined with gas chromatogram-mass spectrometry (GC-MS) technology. Variance analysis, principal components analysis (PCA) and partial least squares-discriminate analysis (PLS-DA) were used to discover which components were affected most significantly by the cluster thinning.【Results】The actual cluster number was not totally consistent with the expected amount. In the same field, the fruit output was increased with the amount of clusters. And the cluster thinning treatments did not cause the different effect on berry weight. The CT1-treated grape berries, compared to the CT2, showed statistically higher total soluble solid content in the Yuanyichang, but the oppisite situation was shown in the Guangdongdi. Titratable acid content displayed no significant difference between the two cluster thinning treatments in Yuanyichang, while it was higher in the CT1-treated grape berries in the Guangdongdi. A total of 31 free-form isoprene volatiles were detected in the developing‘Cabernet Sauvignon'grape berries in this study, including 19 terpenoids and 12 C13-norisoprenoids. Additionally, 17 glycosidically-bound isoprenes were identified including 15 terpenoids and 2 C13-norisoprenoids. The composition and the evolution pattern of these isoprene-derivated volatiles were not altered by cluster thinning treatments, but their concentration was influenced to different extents. Most of terpenoids and C13-norisoprenoids, in particular the glycosidically bound volatiles, were obviously elevated under CT1 treatment relative to CT2.Among them, the most significant increase was found in free-form E-β-damascenone and glycosidically bound α-terpineol, cis-furan linalool oxide and trans-furan linalool oxide. Meanwhile, it was also found that the positive effects of CT1 treatment were more markedly in the vineyard of Guangdongdi, compared to that of Yuanyichang. Furthermore, both terpenoids and C13-norisoprenoids were shown to be higher in the ripening grape berries from the Guangdongdi vineyard. It was thus inferred that the activation of the isoprene metabolic pathway should be attributed to relatively abundant organic matters and mineral elements in this Guangdongdi vineyard.【Conclusion】The viticultural operation of one cluster per shoot can result in the increased concentration of floral/fruity components of grape berries in the regions like Xinjiang, which consequently improves the aroma quality of the wines.