## 4 Discussion  
During the long-term process of pears breeding/selection, great importance has been attributed to fruit size, color, yield, the sugar-acid ratio, and maturation season [37]. This study analyzed changes in the content of sugars and organic acids. As the fruit matured, the sugar content and the sweetness of the fruit increased and the organic acid content and the acidity decreased, which are in line with variations in the ripe flavor of other fruits [38]. In addition to flavor studies, such as those described for sweetness and acidity, aroma is also an important component of fruit flavor. Therefore, elucidating the metabolic pathways controlling the formation and enrichment of key aroma flavor substances is critical in increasing fruit flavor quality. Each fruit species has a unique aroma based on the blending of fruit VOCs [39]. Our study showed that terpenoids and esters were the main parts of the 'Panguxiang' fruit aroma, which is consistent with the main VOCs in the growth and development stages of pineapple (_Ananas comosus_ (L) _Merr._) [40] and 'Nanguo' pear (_Pyrus usuriensis Maxim._) [41]. We analyzed DEGs and differential VOCs in the four stages of 'Panguxiang' and found that the two jointly enriched the terpenoid biosynthesis pathway. Thereby, we boldly speculate that the unique aroma of 'Panguxiang' is most likely due to the increase in the type and content of terpenoids. At present, esters, alcohols, and aldehydes are considered the main volatile aroma components in pears [42]. In our study, however, volatile terpen are second only to esters. Terpenoids have been studied in citrus fruits, and the results show that the volatile oil is mainly composed of monoterene in citrus lemon peel and of terpene and terpene alcohol in oranges [43]. In addition, terpenoids were also reported in the aroma of apricots [44] and peaches [45] during fruit growth and development. On this basis, we further analyzed the biosynthetic pathways of terpenoids to find what interesting results will emerge. As one of the most important groups of plant volatiles, terpenoids are widely present in higher plants and participate in a wide range of biological activities [46]. We built a colorful model for studying 'Panguxiang' terpenoids by comparing stages S1 versus S4 (Figure 4). The model clearly showed how terpenoids were generated, with monoterpens and diterpenes produced via the MAP pathway and sesquiterpenes generated via the MEV pathway [47; 48]. Interestingly, the expressions of biosynthetic genes of sesquiterpenes and mono/di-terpenes were diametrically the opposite. The sesquiterpene biosynthetic gene was significantly up-regulated, while mono/di-terpenes were significantly down-regulated. Furthermore, we identified 15 sesquiterpenes and 7 monoterpenes but no diterpenes. The contents of sesquiterpenoids and metabolites were higher compared with monoterpens. These results indicate that sesquiterpenoids play a key role in volatile terpenoids in the 'Panguxiang' pear, which are mainly produced through the MEV pathway. The terpen accumulated in plants not only contribute to the production of specific aromas beneficial to the plant but are also directly or indirectly involved in the defense against insects or bacteria [49]. Among the metabolic pathways of fruit growth and development, it has been reported that ABA associated with maturity and color-related carotenoids occupy an important position [50]. The color of fruits has always been the target of artificial breeding, and carotenoid production is also through the MEP route. Pears have less color change during growth and development, so we boldly speculate that the content of carotenoids may be reduced. This is also the focus of our next research, which will focus on the changes in the relevant metabolic pathways during maturation. Research on the volatile aroma of pear began in 1927 [6], and a number of related studies are reported every year. However, there are few reports on volatile terpenoids.