# The Influence of UV on the Production of Free Trepenes in _Vitis vinifera_ cv. [PERSON]
## 3 Results and Discussion
### Response of Terpene Compounds to UV-attenuated Treatments
#### 3.2.1 Effects on the Concentrations of Free Monoterpene Compounds  
Overall, higher concentration of total free monoterpene was observed in control compared to TW PC and TW PETG treatments in vintage 2016, but not in 2017. Among all the detected monoterpenes, major differences were observed in \(\gamma\)-terpinene and linalool (Figure 3A). Significantly higher concentrations of \(\gamma\)-terpinene and linalool were observed in TW PETG treatment compared to other treatments in vintage 2016, and a similar trend was also observed in vintage 2017. The application stage of UV attenuation films influenced the concentration of \(\gamma\)-terpinene and linalool at harvest (Figure 4). UV attenuation from veraison to intermediate ripeness (TI) significantly reduced both monoterpenes at harvest compared to application before veraison, whereas the UV attenuation from intermediate ripeness to harvest (TH) increased \(\gamma\)-terpinene content in vintage 2016 but reduced that in vintage 2017. Compared to the pre-veraison (TV) treatment, UV attenuation post-veraison tended to reduce linalool concentration at harvest for both vintages. Application of the PC film at specific stages during ripening resulted in a lower concentration of \(\gamma\)-terpinene compared to the PETG film in vintage 2016, but no differences in vintage 2017. Figure 3: Comparison of key grape terpen in vintages 2016 and 2017. One-way ANOVA was conducted to compare the concentration of (**A**) free monoterpenes, (**B**) free norisoprenoid and (**C**) sesquiterpene between control and TW treatments. * represents significant difference at \(p<0.05\). Figure 4: Comparison of key grape terpenes at harvest treated with UV attenuation at different phenological stages in (**A**) vintage 2016 and (**B**) vintage 2017. Two-way ANOVA was conducted to compare grape terpenes between different UV attenuation sheets (A, B were used to illustrate the statistically significant differences at \(p<0.05\)) and applied stages (a, b, c were used to illustrate the statistically significant differences at \(p<0.05\)). A previous study reported elevated monoterpenes in grapes with enhanced UV radiation [16], which was consistent with the observation between control and TW PC treatment in the current study. [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] [15] found that the monoterpene synthase in the grapevine leaves could be promoted by UV radiation, and the monoterpene production in response to UV radiation follows a similar pattern between leaves and grape bunches [19]. The same study reported elevated monoterpenes at pre-harvest by moderate UV exposure compared to UV attenuation; however, this difference in monoterpenes disappeared at harvest. This was due to the fact that terpene synthase genes could express at later ripening stages after veraison and thus were upregulated by UV radiation during this period [15; 31]. This could explain the reduced \(\gamma\)-terpinene and linalool in UV-attenuated grapes from veraison to intermediate harvest observed in this study. Additionally, the glycosylation of monoterpenes progresses toward harvest, where \(\beta\)-p-glucosyltransferases, as an upstream pathway enzyme, could intervene in the conversion of free terpen to glycosidic forms [32]. As a result, the impacts of UV on free terpen in grapes at harvest may not be significant as was shown in previous and current studies [19; 33]. UV radiation may not always promote the biosynthesis of monoterpenes, but it may depend on the type of monoterpenes and the intensity of radiation. [PERSON], [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] [29] reported that \(\beta\)-d-glucosyltransferase genes _VvGT7_ and _VvGT14_ in grapes (responsible for the production of glycosylated myrcene, nerol, geraniol and \(\beta\)-ocimene) could be upregulated close to harvest when light was completely excluded from the bunch at post-veraison to harvest period, whereas the gene _VvPNLinNer1_ (responsible for free and bound linalool) was continuously downregulated by light exclusion.