## 3 Discussion  
### Aroma Perception from Dynamic Changes of VACs in Pepper Fruits  
The volatile components and relative content are important reasons for people's unique aroma perception of different types of fruits and vegetables [29]. In this study, we detected more than 140 VACs with different contents in a variety of pepper fruits, indicating that the aroma differences were mainly affected by the relative content of VACs. We found that  
Figure 9: Co-expression module identification based on weighted gene co-expression network analysis (WGCNA). (**a**) Module-trait associations based on Pearson correlations. Each row represents a module, and the number of genes in each module is shown on the left. Each column represents a volatile-based aroma compound (“*” represents the significance between groups, the more the number, the more significant). (**b,c**) Heatmaps of genes in modules Dark-olive-green (**b**) and light-yellow. (**c**) Red represents high expression and blue represents low expression. (**d**) Gene co-expression network of the Dark-olive-green module. Co-expression networks were constructed using the top 150 genes based on module membership (kME) values. (**e**) Gene co-expression net-work of the light-yellow module (The red triangles represent the key hub genes in the module).  
HDL1 (Hainan Huangdenglong pepper) and HDL2 (hybrid Huangdenglong pepper) are both processed varieties of chili sauco, and their genetic relationship is similar. However, HDL1 mature fruit has a higher content of flavor components, and the accumulation pattern of HDL2 flavor components is the opposite, which may be the reason why its chili sauco is more popular with consumers. Studies have shown that straight and branched-chain esters can give _Capsicum chinense_ pepper fruits a solid fruity/exotic aroma [23; 30]. In our study, esters were the most critical aroma components in four pepper varieties, similar to the results of previous studies on Cachucha peppers [23], Habanero pepper [31], and Tabasco pepper [32] and accumulated in the late stage of most fruit development in HDL1 and GJ (ghost pepper), which is consistent with the accumulation pattern of fruity esters [33; 34], further explaining the cause of the processing aroma of HDL1 mature fruit. However, the accumulation of esters gradually decreased during HDL2 and increased first and then decreased during CTJ (Chaotian pepper) fruit development, indicating the accumulation pattern difference of esters in different cultivars and developmental stages. The content of esters in HDL 1 mature fruit was significantly higher than other varieties at the same stage, which may be an important reason for for its rich aroma. Aldehydes were the main aroma components at the green stage of pepper fruits and showed a decreasing expression trend with fruit development, which may be related to the gradual transformation of aldehydes into esters with fruit ripening. Terpenes, showing floral and fruity aroma in tea, were detected in pepper fruit in this experiment, and the accumulation pattern was affected by varieties and development stages as well as esters. Although their concentration is usually low, these compounds usually have a low aroma threshold [35], which has an important impact on the aroma quality of pepper.