## 2 Results and Discussion
### Free Volatile Monoterpene Compounds from Berries Treated with TDZ or CK  
Aroma compounds can exist in a free volatile form as well as a glycosidically bound/odorless form in grape berries. The Muscat berry aroma contains higher levels of both free and bound aroma compounds [30]. Monoterpenes are the main volatile compounds in Shine Muscat grape berries, which are responsible for producing the characteristic Muscat flavor of berries [3; 31]. Results presented in this work determined the concentrations of the free and bound monoterpenes of the control and TDZ-treated berries collected at the time of harvest during both sampling periods (2018 and 2019) (Table 1). For the free terpene volatile fraction, a total of 13 compounds were identified and quantified. In 2018 and 2019, the total volatile compounds in CK were generally 3- and 4-fold greater than the TDZ-treated berries, respectively. For the two consecutive years of study, the most abundant free monoterpene aromas in berries were linalool, geraniol and d-limonene. During both years of study, almost all of the monoterpenes showed a significant decrease in berries that were treated with TDZ. Among these, linalool, geraniol and d-limonene were the most odoriferous and abundant monoterpenes.  
Figure 1: Top: Physico-chemical characteristics of grape berries at three different phenological stages of grape ripening with or without thidiazuron (TDZ) treatment over the 2018 and 2019 growing season. Mean \(\pm\) Standard error (\(n=3\)). Different letters indicate significant differences ([PERSON]’s multiple range test, \(p\leq 0.05\)). CK: control, TDZ: thidiazuron. Bottom: comparison of CK and TDZ appearance (28 th Aug., 2018).  
Specifically, the respective reduction of these compounds in the 2-year study of the TDZ-treated berries were as follows: 68%, 89% and 69% (2018), and 74%, 96% and 73% (2019). After treatment with TDZ, a remarkable decrease was also recorded for \(\alpha\)-Terpineol, Citronellol, \(\beta\)-Myrcene, \(\alpha\)-Citral and Neral. These compounds were also detected in other grape cultivars and are known to influence the Muscat flavor of berries [4]. Among the monoterpenes (Table 1), only the concentration of citronellal increased after the TDZ treatment, which is most likely due to the conversion of citronellal to citronellol. Importantly, these findings were in agreement with previous reports, where the most abundant monoterpenes were characterized in Shine Muscat berries [4; 31] and monoterpenes concentrations of berries were reduced after application with a TDZ-like hormone [3].  
Many volatile aroma compounds have been identified in grape berries but the predominate compounds contributing to the aroma profiles of Muscat grape berries were represented by the monoterpenes [5]. Interestingly, almost all of the major free monoterpenes' volatiles detected in our studies were found to be lower in the TDZ-treated berries compared with those treated with CK. The lower levels of monoterpenes in the TDZ-treated berries may eventually result in a lower quality in the flavor and aroma of the grape. To the best of our knowledge, limited studies have been conducted that measure the aroma of TDZ-treated table grapes during fruit development. Previously, the mode of action of TDZ was compared to CPPU (N-(2-Chloro-4-pyridyl)-N'-phenylurea), which shares a similar functional role for reducing the complexity and contents of fruit volatiles [32; 33].