## 3 Results and Discussion
### Effects of Cluster Thinning on Volatile Monoterpene Compounds in Grape
At E-L 37, the total glycosylidically bound monoterpene contents in grapes from the L treatment (1921.40 ug/L) were significantly higher than those receiving the M (1656.36 ug/L) and H (1525.55 ug/L) treatments. From E-L 36 to E-L 37, most of the bound monoterpene profiles such as those of nerol oxide, linalool, neral, geraniol, and geranic acid increased rapidly in the L-treated grapes. The contents of most bound monoterpene profiles were relatively stable from E-L 37 to E-L 37.5, among treatments. At harvest, all the bound monoterpene components, except trans-isogeraniol, were highest in L-treated grapes compared with M- and H-treated grapes. For total monoterpenes (the sum of total free and total bound), the changes were similar to those of the total bound monoterpenes before harvest (Figure S2). At harvest, there was no significant difference between the M (3652.27 ug/L) and H (3723.95 ug/L) treatments, but their contents were significantly lower than that of the L (4527.54 ug/L) treatment. Terpenes, which are crucial determinants of grape and wine aroma quality levels, are influenced by crop-level modifications [34]. The terpene contents are not greatly modified by crop-level modifications [34]. The terpene contents are not greatly modified by crop-level modifications [34]. Figure 1: Influence of cluster thinning on the content of free and bound monoterpenes. (**a**) Heat map of free monoterpene compounds after sucrose treatment. (**b**) Principal component analysis (PCA) of free monoterpenes. (**c**) Heat map of bound monoterpene compounds after sucrose treatment. (**d**) Principal component analysis (PCA) of bound monoterpenes. during the fermentation processes; consequently, their presence in the grapes is directly correlated with their presence in the resulting wine [35]. There have been a number of reports in grapes that focused on the effects of cluster thinning on phenolic compounds but not on aroma composition, particularly in table grapes [5]. Cluster thinning significantly increases the monoterpene and sesquiterpene contents in 'Syrah' wines [36]. In 'Sauvignon Blanc' berries, the free and bound terpene profiles are significantly increased by cluster thinning performed one week before veraison [8]. Here, using 'Muscat Hamburg' grapes, severe cluster thinning (L and M treatments) significantly enhanced the total monoterpene concentration compared with a lesser degree of cluster thinning (H treatment, similar to the average load of the control), consistent with previous results [6; 36]. Thus, a higher intensity of cluster thinning (L treatment) resulted in the highest total monoterpene level. Consistent with our results, [PERSON] et al. [1] indicated that a more intense thinning treatment resulted in higher monoterpene concentrations in 'Pinot Noir' wines compared with a less intense thinning treatment. [PERSON] et al. [37] also reported that high-intensity cluster thinning produced higher anthocyanin concentrations than low-intensity cluster thinning. This might be because cluster thinning allows more light exposure on the remaining berries, which influences monoterpene concentrations [1]. A PCA was performed to find patterns in the variances of monoterpene profiles from the 54 samples analyzed (six developmental stages \(\times\) three treatments \(\times\) three biological replicates) (Figure 1b,d). There were obvious distinctions in the free monoterpene profiles among different stages and treatments, and they were largely influenced by PC1, with a 55.47% contribution rate. The contribution rates of PC2 and PC3 were 16.69% and 10.49%, respectively. For the bound monoterpene, only the maturity period was distinguished from the other periods.