## 3 Results and Discussion
### Effects of Cluster Thinning on Volatile Monoterpene Compounds in Grape  
Because of the pleasant Muscat-flavor of 'Muscat Hamburg' grapes, the effects of cluster thinning on aromatic compounds, particularly monoterenes, during the ripening were investigated (Figure 1a,c; Tables S1 and S2). To explore the differences in monoterenes among treatments, grapes were sampled six times at E-L 35, E-L 35.5, E-L 36, E-L 37, E-L 37.5, and E-L 38 during growth and development. Monoterpenes exist in both free and bound forms in grapes. Free form monoterenes are the crucial and direct contributors to the floral and fruity odors of grapes and wines [20]. In total, 27 free monoterenesprofiles were detected, but trans-rose oxide, cis-isogeraniol, and trans-isogeraniol were not detected during the early period (Table S1). At the beginning of the veraison (E-L 35), the total free monoterpene contents ranged from 138.86 ug/L (L) to 148.92 ug/L (H). There were no significant differences in the cis-rose oxide, neral, \(\beta\)-Citronello, \(\gamma\)-geraniol, and nerol contents among treatments. The monoterpene contents, except linalool and geraniol, were highest in the H treatment. In E-L 35, geraniol and geranic acid were the most abundant monoterpenes. At E-L 35.5, the total free monoterpene contents increased by 36.45%, 27.00%, and 16.26% in the L, M, and H treatments, respectively, compared with those in E-L 35. The higher total free monoterpene content in the L treatment resulted from the higher linalool content. When veraison was completed (E-L 36), the total free monoterpene contents were 245.61, 343.72, and 321.40 ug/L in the L, M, and H treatments, respectively, and linalool, \(\beta\)-myrce, limonene, and \(\alpha\)-terpineol synthesis began. The remaining free monoterpene profiles, except those of cis- and trans-rose oxide, were lowest in the L treatment, with the M treated-grapes having higher contents. At E-L 37, the linalool content was highest in the L treatment, and the differences in the rest of the free monoterpenes among treatments were similar to those in the E-L 36 period. At this stage, linalool was most abundant, followed by geraniol and geranic acid. At E-L 37.5, the total free monoterpene content was highest in the L treatment compared with the H and M treatments due to the higher linalool, geraniol, and \(\alpha\)-terpineol contents. Most of the free monoterpene profiles decreased in the H treatment compared with the L and M treatments. At harvest, significant increases in the levels of most free monoterpenes were observed in all treatments, and the total free monoterpene content reached its maximum level. Similar to E-L 36 and E-L 37, when moderate numbers of fruit remained, they had higher free monoterpene contents. Among the free components, linalool and geraniol were the most abundant free monoterpenes, followed by \(\alpha\)-terpineol, \(\beta\)-myrcene, \(\beta\)-cis-ocimene, and nerol, which are all crucial contributors to the floral aroma of 'Muscat' varieties [32]. Bound-form monoterpenes are potential contributors to grape odors, and they can be converted into free-form monoterpenes by hydrolysis during berry development, which increases the aromatic characteristics [33]. The bound monoterpenes in six developmental stages of 'Muscat Hamburg' are shown in Figure 1c and Table S2. The total concentration of glycosylated bound monoterpenes was lowest in the L treatment at the early stage of grape development (E-L 35, E-L 35.5, and E-L 36) and highest in the L treatment at the later stages. At E-L 35, the abundant glycosylidically bound profiles were those of geraniol, geranic acid, nerol oxide, and nerol. Trans-rose oxide, cis-isogeraniol and trans-isogeraniol were not detected at these stages. Most of the bound monoterpene profiles were highest in the M treatment. The variation trends of bound monoterpenes among treatments at E-L 35.5 and E-L 36 were similar to those of the last period, with most of them being highest in the M treatment, followed by the H treatment and then the L treatment.