## 2 Results and Discussion  
### VOC Metabolites in Five Rice Varieties  
Three well-known aromatic rice varieties from different rice-producing areas in China were chosen as research objects. These varieties are representative of each region and are all gold prize-winning, high-quality rice varieties [6]. HS-SPME-GC-MS was used to detect the VOC metabolites in all rice samples. A total of 656 VOC metabolites were identified and quantified. In a previous analysis of rice VOCs, [PERSON] et al. [10] found that the aroma volatiles usually included an oxygen-containing group, a nitrogen group, a sulfur group, and an aromatic group. In this study, the identified species of VOC metabolites were mainly lipids,lipid-like compounds (i.e., hydrocarbons, alcohol and aldehyde), benzenoids, and organic oxygen compounds. To characterize the overall metabolic differences among the five rice varieties, as well as the variability among samples of each individual variety, the principal components of all samples were classified by similarity. The PCA of the five varieties is shown in Figure 1a. Two principal components cumulatively accounted for 46.36% of the total variation, with PC1 explaining 29.16% and PC2 explaining 17.2% of the variance. The replicate samples of each variety clustered together, forming five groups, but the five varieties were quite dissimilar. This shows that the growth environment of the rice variety, such as climate, soil conditions, and altitude, strongly influences the metabolite accumulation of the rice grain [10]. Indeed, previous authors have indicated that metabolite accumulation is substantially affected by environmental factors as well as by genetic factors [3]. Figure 1: The differentiation of VOC metabolites in five rice varieties. (**a**) PCA analysis of five rice varieties. The PC1 of \(X\)-axis and PC2 of \(Y\)-axis in the figure represent the scores of the first and second principal components respectively. The scattered points of different colors represent the samples of different rice varieties, and the ellipse is the 95% confidence interval. (**b**) Venn diagram of VOC metabolites in five rice varieties. (**c**) Box and whisker plots of the relative level of acetoin in five rice varieties. (**d**) Box and whisker plots of the relative level of 2-heptanone in five rice varieties. Dao stands for the variety of Daohuaxiang 2, Ye stands for the variety of Yexiangyoulisi, Mei stands for the variety of Meixiangzhan 2, Hu stands for the variety of Huanghuazhan, and Pu stands for the variety of Yanfeng 47. To better understand the main substances that differed among the rice varieties, the 30 VOC metabolites with the highest abundances in each rice variety were selected for subsequent analysis. The selected VOC metabolites with their relative abundances across all the cultivars were illustrated in Table 1. A Venn diagram of these VOC metabolites showed that 17 metabolites were shared among all five rice varieties: 1-hexanol, fluoromethyloxirane, 1-butanol, 1-pentanol, dimethylsilanediol, acetone, acetic acid, hexanal, 1-octen-3-ol, 1-penten-3-ol, 2-pentylfuran, dibutyl phthalate, hexanoic acid, 1-heptanol, 2,4-dimethylbenzaldehyde, ethyl acetate, and decamethylcyclopentasiloxane (Figure 1b). These 17 metabolites were mainly alcohols and heterocyclic compounds. Alcohols in the subclass fatty alcohols, such as 1-hexanol, 1-butanol, 1-pentanol, 1-octen-3-ol, 1-penten-3-ol, and 1-heptanol, are the secondary products of polyunsaturated fatty acids and produce a soft smell [10]. Across all rice varieties, 1-octen-3-ol was the most abundant VOC; this compound produces an odor of mushrooms and straw [10]. Other abundant VOCs were 1-hexanol, contributing a grassy herbaceous and sweet flavor [15], and 1-butanol, which is produced via the degradation of aromatic compounds and has a floral smell [16]. As for heterocyclic compounds, such as furans, their primary pathways are associated with lipid oxidation or the Maillard reaction, and produce a caramel-like odor [10].