Effect of vine shading on the accumulation of norisoprenoids volatiles in 'Cabernet Sauvignon' grape berries

Abstract：【Objective】Norisoprenoids are considered as the main components that contribute to grape varietal aroma and eventually determine the flavor quality of grapes and wines, due to their extremely low perception threshold and pleasant floral fruit characteristics. These compounds can be generated by direct degradation of carotenoids, and exist as both free-form and glycosidically-bound form. Sunlight is the most important environmental factor for plant growth and development and affects the accumulation of norisoprenoids in grape berries. However, deficient or excessive sunlight will probably cause negative effects on the accumulation of norisoprenoids. Xinjiang is a vital 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 large temperature difference between day and night, which appears to be unsuitable for the accumulation of volatile compounds such as norisoprenoids. However, the effect of vineshading on norisoprenoids production under this particular climate remains to be poorly understood.The present study aimed at assessing the impact of vine shading on norisoprenoid concentration and the expression of related genes in Vitis vinifera'Cabernet Sauvignon'grape berries. The outcome provides a theoretical support and practical reference for improving the quality of grape and wine in the regions with the similar climate condition of Xinjiang.【Methods】The vine shading treatments were conducted in the vineyard of the Rhyme manor located in Changji county, Xinjiang. All the grapevines were planted in 1999 and were managed from north to south. The row and vine spacing was 2.5 m × 1.0 m, and each vine was allowed to bear approximately 10 to 15 berry clusters. According to the shading level, the shading treatments with black shading nets were divided into Net 1 (reducing about 50% of sunlight intensity around the cluster) and Net 2 (reducing about 20% of sunlight intensity around the cluster) .Shading nets were set up 10 cm above grapevine canopy at pre-veraison (Q1 and Q2) and post-veraison (H1 and H2) , and the shading treatments lasted till harvest. Non-shading grapevines were taken as the control. The randomized block design was adopted, and all the treatments were carried out with three biological replicates corresponding to three independent blocks. The free and glycosidically-bound norisoprenoids in grape berries were qualitatively and quantitatively analyzed using the head space-solid phase microextraction combined with gas chromatogram-mass spectrometry technology (HSPM-GC/MS) . The expression amount of the genes related to norisoprenoid biosynthesis was measured by Realtime q-PCR using SYBR green method on an ABI 7300 Real-Time System. The differences in the norisoprenoid accumulation and the transcript abundances were compared between the shaded and control grape berries.【Results】The sunlight intensity around the clusters was decreased significantly by shading treatment, but the mean temperature and relative humidity were hardly affected. As a consequence, all the four vine shading treatments markedly reduced total soluble solid content, but had little impact on titratable acid level in ripe berries. A total of 11 nisoprenoid components were detected, comprising11 glycosidically-bound components and 7 free-form ones. Both Q1 and H1 treatments significantly increased the total content of norisoprenoids, among which β-damascenonet was raised largely. By contrast, Q2 and H2 treatments had no obvious change compared to the control. The analysis of Ortho Partial Least Squares-Discriminate Analysis (OPLS-DA) revealed that the Q1 and Q2 treatments mainly influenced the components such as 6-methyl-5-hepten-2-one, (E) -β-damascenone and (Z) -β-damascenone, while the H1 and H2 treatments more altered the components like TPB and TDN. Additionally, the Pearson's correlations between the production of metabolites and the expression abundance of the related genes were analyzed. The results indicated that the expression level of VvPSY2 was positively correlated with the concentrations of β-cyclocitral, geranylacetone and (E) -β-ionone, whereas the expression of VvCCD1, VvCCD4 a and VvCCD4 b were negatively associated with the concentrations of most norisoprenoid components. Moreover, there was a low Pearson's correlation coefficient between VvPSY1 expression and norisoprenoid production. Q1 and H1 treatments significantly down-regulated the expression of VvPSY1 and VvCCD1, while VvCCD4 a and VvCCD4 b were up-regulated significantly by Q2 and H2 treatments at ripe stage.【Conclusion】In the grape-producing regions with strong sunshine and little rainfall, moderate shading that reduces sunshine intensity can improve the aroma quality of grapes and wines.