## 2 Results and Discussion
### Partial Least Square Discriminant Analysis  
Based on 83 volatile variables in the grapes at harvest of 2010 vintage, partial least square discriminant analysis (PLS-DA) was applied to sort out individual volatile components that are responsible for differentiation amongst these various treatments. The data were normalized by the Autoscaling method in the MetaboAnalyst 3.0 [42]. The reliability of the discrimination test was evaluated by Leave one out cross-validation (LOOCV) method (Supplementary Figure S1). As shown in Figure 6A, these treatments were clearly segregated from each other, and the semi-transparent fields meant 95% confidence intervals. Variables with VIP score \(>\)1.5 were considered important contributors for the segregation (Figure 6B). Fifteen major components were screened out, including eight benzenoids, three straight-chain aldehydes, three straight-chain esters, and one C13-norisoprenoid. Of the selected benzenoids, four benzenoids were detected only in the UV attenuation treated grapes at harvest, contributing to the separation between all the UV attenuation treatments and the control. Of them, \(p\)-Cymene, 3-ethyl benzaldehyde, and (\(E\))-cinnamaldehyde were only present in the UV3-A and the UV3-B treated grapes, whereas 4-methyl benzaldehyde existed in all the UV-attenuation treated grapes but not in the control (Table 1). It is known that the grape maturation stage is crucial for the accumulation of benzene derivatives in grapes [9]. This might explain why the attenuated UV radiation during grape mature period (UV3-A and UV3-B treatments) showed a much more significant promotion on the accumulation of benzenoids. Additionally, the concentration of styrene increased considerably in the grapes with UV attenuation treatments (Figure 6B). About a 3.5-fold enhancement of styrene was observedin the grapes treated by the UV1-A and the UV1-B. On the contrary, both benzyl alcohol and \(\beta\)-phenylethanol concentrations were significantly reduced in the UV3-treated grapes. Also, \(\alpha\)-phenylethanol level was markedly lower in the other UV-treated grape berries except for the UV1-A treatment, and the greatest decrease was found in the UV3-A and the UV3-B treated grapes (Table 1). The alteration of these compounds resulted in the difference in the total concentration of amino acid-derived volatiles between the UV treated grapes and the control (Figure 4). The odor thresholds of benzenoids in grapes tend to be higher than that of other classes of flavor and aroma compounds, like norisoprenoids or methoxypyrazines. From Table 1, it was observed that the concentrations of benzenoid compounds varied from trace levels to hundreds of micrograms per liter, indicating that these chemical species might contribute to the overall odor of juice from these grapes [43]. Figure 6: Results of PLS-DA. (**A**) 2D scores plot. The corresponding interactive 3D plot is shown in its top right corner; (**B**) Selected volatile compounds based on VIP scores. The attenuated UV radiation around the fruit zone substantially decreased the concentrations of three straight-chain aldehydes in the grapes: (_E_)-2-hexenal, (_E_)-2-heptenal and heptanal. The lowest abundances were all observed in the UV1-B treated grapes (Table 1). These three compounds were found to distinguish the attenuated UV-treated grapes from the control. In grape berries, (_E_)-2-hexenal was the most abundant aldehyde, accounting for 50% of total concentration of straight-chain aldehydes. The influence of UV radiation reduction on this compound eventually caused the difference in aliphatic aldehydes between the UV treated and the control grapes (Figure 5). In grapes, straight-chain aldehydes or alcohols are described as green or leaf-like odors that provide an undesirable flavor perception. Therefore, the attenuated UV radiation might help to eliminate these off flavor attributes. Both 2-butoxyethyl acetate and (_Z_)-3-hexen-1-ol acetate levels were largely decreased in the grapes treated with the attenuated UV radiation.