## 2 Results and Discussion
### Effect of Attenuated UV Radiation on Three Major Classes of Volatiles
#### 2.1.2 Effect on Amino Acid-Derived Volatiles  
Amino acid-derived volatiles include benzenoids, branch-chain aliphatics, and methoxypyrazines (Figure 1). Phenylalanine is formed from shikimate acid pathway and further metabolized to yield benzenoids. Twenty benzenoids were identified in the 2010 vintage grapes (Supplementary Table S1). Benzoic acid ethyl ester, naphthalene, and 2-methoxy-phenol were only detected in the grapes at harvest. The total concentration of benzenoids showed an increasing trend during the berry maturation, and reached the highest level at harvest (Figure 4). The similar pattern was also reported by [PERSON] _et al._ (2009) in Cabernet Sauvignon grapes [9].  
Although the concentration of benzenoids in the UV1-B treated grapes at 8-waf was lower than the control, all the UV attenuation treatments resulted in an enhancement in the concentration of benzenoids at harvest. The UV1-A, the UV1-B (UV attenuation from fruit-set to the harvest), and the UV2-B treatments (UV attenuation from fruit-set to the onset of veraison) caused a more rapid accumulation of  
Figure 4: Effect of attenuated UV radiation n the concentration of amino acid-derived volatiles in developing “Cabernet Sauvignon” grapes. For more information, please refer to the explanation in Figure 2.  
benzenoids during grape maturation, and eventually led benzenoids to a higher level at harvest compared with the control (Figure 4). The other three treatments (UV2-A, UV3-A, and UV3-B) also enhanced the benzenoid level although they were not statistically significant. The enhancement in the concentration of benzenoids was the greatest in the UV1-B treated grapes, followed by the UV1-A and the UV2-B grapes. These suggested that the formation of benzenoids might be intensified with the intensity reduction of solar UV radiation. This activation might be related to the action of benzenoid volatiles as plant defense molecules [4,34]. The shikimate pathway mediates the flow of carbon from metabolism of carbohydrates to biosynthesis of aromatic compounds in plants. It has been demonstrated that environmental stresses can activate the shikimate acid pathway and further promote the accumulation of secondary metabolites [18,35,36]. In the present study, most of the solar UV components reaching the fruits had been removed, altering the microclimate around the fruit zone and consequently stimulating the accumulation of secondary metabolites including benzenoids in the grape berries.  
Branched-chain aliphatic volatiles and methoxypyrazines in grapes are derived from three branch-chain amino acids: valine, isoleucine, and leucine. Five branched-chain aliphatic compounds were identified in these grapes (Supplementary Table S1). Their total concentration was relatively stable in the whole development stages and the highest level was observed at 8 waf. Subsequently, the UV1-A and the UV1-B treated grapes experienced a gradual decreasing trend in the concentration of branched-chain aliphatic volatiles till harvest, whereas the control and the other treated grapes showed a slight increase in the concentration at pre-harvest. As a result, the branched-chain aliphatic volatiles were present at lower concentrations in the UV1-A and the UV1-B treated grapes. Except this, no significant differences were observed between the control and the treated grapes in any other stages (Figure 4). With regards to MPs, only isobutyl methoxypyrazine (IBMP) was detected in the grapes in the early stage of development (Supplementary Table S1). The concentration of IBMP continuously decreased from fruit-set to the onset of veraison, and reached the trace amount afterwards. At 5 waf, the UV1 treatments significantly decreased the concentration of IBMP in the grapes compared to the control (Figure 4). The studies of [PERSON] _et al_. (2012) [24] and [PERSON] _et al_. (2014) [21] also indicated no significant impact of the reduced UV radiation on IBMP concentration in Sauvignon Blanc grapes and wines.