## 4 Discussion  
Fruit aroma is composed of a complex combination of numerous volatile compounds and represents a major quality attribute of pear fruits, accounting for \(10^{-4}\) to \(10^{-7}\) of fruit mass [19; 20]. Volatile aromatic substances are formed dynamically in fruits, and their types and concentrations change over time, such as D145, XMW1069 and NMW0130, but most of the aromatic substances are formed in later stages, including KMW0580, XMM0088 and KMW0530 [13; 21] (Figure 2). Moreover, the composition of aroma is specific to the species of pear [4]. Volatile aromatic substances in pears have been extensively studied. For example, in four additional cultivars (e.g., 'Xiang-mian Li', 'Mu-tou Su', 'Dang-Shan Suli' and 'NanGuo Li' pear), aldehydes and alkenes are formed during early stages but alcohols and esters gradually increase in abundance as time progresses and the total aroma substances accumulate [21]. Reportedly, 121 metabolites were identified and quantified, including 40 esters, 32 alcohols, 16 aldehydes, 13 alkenes, 11 ketones and 4 acids in 12 cultivars of pear fruits [13]. Moreover, 221 major volatile components were detected in 202 mature pear cultivars, among which aldehydes, esters and alcohols were the most dominant [20]. In the present study, we detected 434 metabolites from 14 types in 'Panguxiang' pears in total, and the majority of these metabolites consisted of esters and terpenoids (Table 1, Table S1). Sugars and aroma are essential quality traits and have induced a great deal of research into the physiological properties of fruit aromas [17]. In pear leaves, sorbitol as the main photosynthetic product is transported through the phloem and accumulates in the fruits. Here, during the growth and storage stages of pear fruits, the sorbitol content is always the highest of all sugars, followed by fructose and glucose in the 'Panguxiang' pear (Figure 3A). In comparison, fructose content is the highest, followed by sorbitol and glucose, in the 'Korle' pear (Figure 4A). Notably, the content of sucrose was lower and gradually declined in both the 'Panguxiang' pear and the 'Korle' pear. However, in the Chinese white pear, sorbitol accounts for the largest proportion of total sugars at the early stage but then declines. In mature pears, sucrose and sorbitol account for about 30% and 20% of total sugars, respectively, with a smaller amount of fructose [17]. Hence, among various sugars, fructose, glucose and sucrose primarily affect the flavor of the 'Panguxiang' pear. Volatile compounds are mainly derived from the metabolism of fatty acids, amino acids and carbohydrates [4; 17]. The availability of primary precursor substrates is crucial for the biosynthesis of these aroma compounds, including sugars, fatty acids and amino acids. In most fruits, the major precursors of aroma volatiles are either saturated or unsaturated fatty acids [17; 22]. Fatty acid-derived straight-chain alcohols, aldehydes, ketones, acids, esters and lactones, ranging from C1 to C20, are important and characteristic aroma compounds that are responsible for the intense flavor of fresh fruits [17]. Within pears, sugars, as precursors, are used in the biosynthesis of many quality compounds and flavor components, such as vitamins and aromatic substances [17]. The main types of volatile aromatic substances formed through carbohydrate metabolism are furaneols, pyrones and various terpen [17]. Moreover, esters, terpen, and aldehydes were the