## 1 Introduction  
High-quality grapes usually produce low to moderate yields, and over-cropping reduces grape and wine quality level [1]. Cluster thinning is a vineyard practice used worldwide to regulate vine yields by removing whole-grape clusters [2]. This practice results in improved conditions for the retained grapes by changing the source/sink ratio and the leaf area/fruit weight ratio, thereby regulating secondary metabolite synthesis and accumulation [3, 4]. The increased leaf area is beneficial to photosynthesis, which promotes berry development and maturation [1]. The change in the source/sink ratio resulting from cluster thinning enhances the accumulation of secondary metabolites such as anthocyanins and favorablys [5]. Cluster thinning affects berry ripening, which influences the acid, sugar, aroma, and polyphenol contents of the harvested grapes and resulting wines [6, 7]. Cluster thinning significantly increases the free and glycosylated terpene content in 'Sauvignon Blanc' berries [8]. The impact of cluster thinning on secondary metabolites in berries is affected by the cultivar, vintage, the amount of berry removed. and the operation timing [3, 9]. Cluster thinning needs to be targeted in different grape varieties to optimize grape components [4]. [PERSON] et al. [2] reported that cluster thinning reduces sensory attributes in 'Tempranillo' wines, but promotes them in 'Grenache' wines. Cluster thinning increases the anthocyanin contents in 'Cabernet Franc', 'Sangiovese', 'Cabernet Sauvignon', and 'Merlot' [10, 11, 12], but has had inconsistent results in 'Pinot Noir' [13]. The timing of cluster thinning is also debatable. [PERSON] et al. [3] and [PERSON] et al. [14] indicated that cluster thinning negates the desired results when applied at earlier stages such as at bloom. This may indicate that lower leaf photosynthesis rates result from the reduced sink sizes, which leads to a lack of sugar being available for the remaining berries [3; 15]. In addition, [PERSON] et al. [16] reported that berries from vines thinned at early stages had higher sugar levels and lower tittatable acidity levels than those thinned at later stages. The lag phase in grape ripening is considered the appropriate time for thinning because the lag-phase cluster weights are used to estimate yields [9]. When thinning is conducted at veraison, its influence on cluster size and berry weight is lower than when it is implemented soon after fruit set [17]. Cluster removal during bloom promotes berry size in 'Cabernet Sauvignon', 'Pinot Noir', 'Riesling', and 'Chenin Blanc' grapes [13; 14]. In addition, vine watering regimes and climatic conditions also affect the effectiveness of cluster thinning [14]. Lower yields in a vineyard do not always lead to high wine quality levels, particularly in warm-weather vintages [18]. Terpenes are the most important class of aroma components, and they contribute to the pleasant flavors including the floral, rose-like, green, coriander, and citrus sensory characteristics of grapes and wines [19; 20]. Monoterpenes are more abundant than hemiperenes, sesquiterpenes, and diterpenes in grapes [21]. The former exists in two forms, free forms, which directly contribute to flavors and aromas, and glycoside conjugates, which are regarded as potential aroma components [22]. Odor-active volatile compounds are released from glycoside conjugates through acidic or enzymatic hydrolysis [23]. The biosynthesis of monoterenes starts with the production of plant isopentenyl diphosphate and dimethylallyl diphosphate in two biosynthetic routes, the primarily cytosolic mexulonic acid and the plastidial methylerythnitol phosphate pathways, with the latter being the main synthetic pathway in grapes [24]. The biosynthesis and accumulation of monoterenes are regulated by several factors such as grape ripening processes, grape varieties, and agronomic practices [21; 25]. Additionally, terpenes are responsive to yield-level modifications [8].