## 2 Results and Discussion
### Effect of Attenuated UV Radiation on Three Major Classes of Volatiles
#### 2.1.1 Effect on Isoprenoid Volatile Compounds  
Isoprenoid metabolism yields terpenes and norisoprenoids (Figure 1). Terpenes are dominant volatiles in the family of isoprenoid derivatives, and determine the evolutionary trend of this volatile class. In this study, 18 terpene compounds were identified (Supplementary Table S1), and their total concentration had a decreasing trend throughout the development stages (Figure 3). The similar variation pattern was also reported previously in Cabernet Sauvignon cultivar [9]. Except that the UV1-B treated grapes showed higher level of terpenes at 5 waf, no significant differences were observed between the control and the treated grapes in any other stages (Figure 3). This suggested that UV radiation attenuation exerted an enhancing effect before veraison but did not impact the level of terpenes at harvest. Similarly, [PERSON] _et al_. (2015) found that terpene level in UV-exclusion-treated Pinot Noir grape wine had no significant difference compared to the control [20]. However, [PERSON] _et al_. (2014) [21] observed that the content of terpenes was significantly lower in attenuated UV-radiation Sauvignon Blanc grape wine. These indicated that cultivar differences might affect UV radiation effect. Most of the terpene compounds are described as fruity-flowery aroma. In neutral varieties like \"Cabernet Sauvignon\", terpen are usually at very low levels with a minimal flavor impact [31].  
Eight norisoprenoids were identified in this study, including \(\beta\)-damascenone, \(\beta\)-ionone, geranylacetone, 5-hepten-2-one,6-methyl-, _cis_-theaspirane, nerylactone, TCH (Cyclohexanone, 2,2,6-trimethyl-), and dihydroudecan I (Supplementary Table S1). During the development stages, the concentration of norisoprenoids continuously decreased in the UV3-A and the UV3-B treated grape berries. However, these volatile levels in the other UV treated grapes and the control showed a decrease during the development stages, but then a slight increase at harvest. The norisoprenoid levels in the UV3-A and the UV3-B treated grapes were lower than the control at harvest (Figure 3). Norisoprenoids in grapes are yielded by photochemical and/or enzymatic degradation of carotenoids in the skin and pulp [32]. The degradation  
Figure 3: Effect of the attenuated UV radiation on the concentration of terpenes and norisoprenoids in developing “Cabernet Sauvignon” grapes. For more information, please refer to the explanation in Figure 2.  
of carotenoids has been reported to be inhibited in grapes under low UV-B radiation [15,16]. As a result, the concentration of carotenoids at harvest was higher compared to that under the ambient UV-B radiation. In the present study, the reduction of norisoprenoids in the UV3 treated grapes might result from the inhibition of carotenoid degradation in the grapes exposed to low UV-B radiation. It should be noted that no significant changes in the norisoprenoid concentration were observed between the UV1 treated and the control grapes even though the UV1 treatment window (from 3 waf to 17 waf) covered the UV3 treatment period. This might be because the most dramatic changes in the composition of grape berries normally occur during veraison or ripening phase. During that time, sugar/acid balance and yields of flavor and aromatic compounds and their precursors play primary roles in determining the flavor attributes of grapes [33]. Therefore, we speculated that compared with the attenuated UV radiation beginning from 3 waf (for example UV1 treatments), the UV radiation attenuation from the onset of ripening (for example UV3 treatment) could cause a stronger response to grapes under the sudden UV stress at this specific time point. In addition to norisoprenoids, some of volatiles mentioned below also displayed more significant modifications in the UV3 treated grapes compared to the UV1 treated grapes.