# 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
#### 3.2.3 Effects on the Concentrations of Sesquiterpene Compounds  
Fifteen sesquiterpenes identified in this study can be classified into four groups according to their biosynthesis pathways to better understand the impacts of UV attenuation. \(\alpha\)-Humulene is formed via the humlyl carbocation pathway, 1-epi-cubenol and cubenol are formed via the nerolidol diphosphate pathway and \(\gamma\)-eudesmol is derived from \((E,E)\)-germacradienyl cation. The rest of the sesquiterpenes are all biosynthesised through the germacrene D pathway [2; 25]. Minimal influences of UV attenuation on the concentrations of sesquiterpenes were observed in both vintages, except for \(\alpha\)-muurolene and \(\gamma\)-cadinene, which account for over 80% of total sesquiterpene contents (Tables S2 and S3). Out of five compounds detected only in vintage 2017, \(\delta\)-cadinene, zonarene, \(\omega\)-cadinene and cubenol were only found in UV-attenuated treatments. Increased \(\alpha\)-muurolene was observed in the UV attenuation treatment in vintage 2016, but not in vintage 2017 (Figure 3C). In contrast, a clear reduction of \(\gamma\)-cadinence was observed in UV-attenuated treatments. Importantly, UV attenuation from intermediate ripeness to harvest tended to significantly increase the concentration of sesquiterpenes at harvest (Tables S2 and S3). This phenomenon was very clear in the case of \(\alpha\)-muurolene and \(\gamma\)-cadinene, where UV-attenuated grape from intermediate ripeness to harvest had the highest concentration of \(\alpha\)-muurolene and \(\gamma\)-cadinence in both vintages (Figure 4). Sesquiterpenes in grapes were present in free forms rather than glycosidic bound forms [2; 25], thus the changes in the concentration of sesquiterpenes were likely due to the influences of UV on their biosynthesis. Previous research studies reported that the concentration of some sesquiterpenes in grape flower such as \(\beta\)-farnesene and \(\alpha\)-panasinsene could be dramatically reduced by UV-B exclusion, while other sesquiterpenes such as farnesol could be increased by UV-B exclusion, and others, such as \(\alpha\)-caryophyllene, \(\beta\)-caryophyllene and nerolidol, remain unchanged [40]. A study on peach also found elevated sesquiterpene, namely \(\alpha\)-farnesene, after 48 h of UV-B radiation due to the upregulation of _PpTPS2_ from a TPS-b terpene synthase subfamily [22]. Despite this, the same study also reported a reduced concentration of valencene in peach after UV-B radiation, while the concentration of \(\alpha\)-cedrene decreased after 6 h of UV-B exposure and increased afterward. These examples in the literature suggest that the impacts of UV radiation on individual sesquiterpenes may vary from each other depending on the extent of UV exposure, and the sesquiterpenes related to plant defence are likely to be upregulated by UV-B [15; 22; 40]. Regarding the impacts of UV attenuation film's application stage, our previous research studies and others have demonstrated that the concentration of most sesquiterpenes decreased from fruit-set to veraison and gradually increased at the late stage of ripening, and many sesquiterpenes only appeared at the end of the grape ripening period [25; 26; 27; 28; 41]. Sesquiterpenes are biosynthesised through distinct biological pathways, and the responses of sesquiterpenes to UV attenuation at different phenological stages could vary among different biosynthesis pathways [2]. In the current study, no clear changes were observed in sesquiterpenes derived from the humlyl carbocation pathway, the nerolidol diphosphate pathway and the \((E,E)\)-germacradienyl cation pathway, whereas the two sesquiterpenes (\(\alpha\)-muurolene and \(\gamma\)-cadinene) derived from the germacrene D pathway had the most obvious changes. Our previous study observed the highest concentration of \(\alpha\)-muurolene before veraison with a decreasing trend afterward until harvest, whereas \(\gamma\)-cadinene could only be detected at harvest [26; 38].