## 2 Results and Discussion
### Multivariate Analyses of Metabolites in the Five Rice Varieties
#### 2.2.1 Variation in Metabolic VOCs among Aromatic Rice Varieties  
The results showed that the three aromatic rice varieties differed greatly (Figure 2a-c), that is, 10 metabolites distinguished Meixianghan 2 from the other varieties: (2-methyloctyl) benzene, 2,2-dichloroethanol, indole, 2,4,6-trimethylpyridine, 2-(octyloxy) ethanol, succin acid but-3-yn-2-yl 2-methylpent-3-yl ester, 2-ethylheptanoic acid, 3-buttene-1,2-diol, dibutoxymethane and 2-octen-1-ol (Table 2). Similarly, 11 differential metabolites were identified between Daohuaxiang 2 and other two aromatic rice varieties: butyl benzoate, DL-2-phenyl-1,2-propanediol, 2-acetyl-1-pyrroline, 1,3-dichloro-2-methyl benzene, sulfurous acid dodecyl pentyl ester, 2-(1-cyclopent-1-enyl-1-methylethyl)cyclopentanone, 2-methylpropanoic acid 3-hydroxy-2,2,4-trimethylpentyl ester, 6,6-dimethyl-cyclohex-2-enem-1-ol, 2-bromocycloheptanone, 3,8-dihydroxy-3,4-dihydronaphthalene-1(2\(H\))-one, and 1-(1\(H\)-pyrrol-2-yl)ethanone (Table 2). Finally, Yexiangyoulisi was distinguished from the other varieties by the metabolites (15-_exo_)- 2-methyl-3-methylene-2-(4-methyl-3-pentenyl)bicyclo [2.2.1]heptane, 2-butyl-2-octenal, 1-ethenylaziridine, 2-methyl- 2-octen-4-ol, and 2- cyclohexylpiperidine (Table 2). No shared metabolites were identified among three aromatic rice varieties. Biosynthesis of secondary metabolites, protein digestion and absorption, and toluene degradation were the main pathways associated with the significantly different metabolites between Yexiangyoulisi and Daohuaxiang 2 (Figure 2d), while the metabolites that differed significantly between Meixiangzhan 2 and Daohuaxiang 2 were mainly associated with the biosynthesis of secondary metabolites, protein digestion and absorption, and aminobenzoate degradation (Figure 2e). The metabolites that differed significantly between Yexiangyoulisi and Meixiangzhan 2 were mainly associated with the biosynthesis of secondary metabolites, protein digestion and absorption, of secondary metabolites, protein digestion and absorption, and ethylbenzene degradation (Figure 2f). In general, the metabolites that differed significantly among the three aromatic rice varieties were associated with the biosynthesis of secondary metabolites and with protein digestion and absorption. The differences in the biosynthesis of secondary metabolites among the three kinds of rice grains were closely related to characteristics of the growth environment, such as climate, soil conditions, and altitude [1]. The geography and climate of the regions producing the three aromatic rice varieties used in this study differ substantially. Guangdong province has a warm climate, sufficient sunshine, abundant rainfall, and high levels of organic compounds in the soil [23]. Water can promote the synthesis of organic acids; these metabolically active solutes participate in osmotic adjustment and help to balance excess cations in plants [24]. Therefore, organic acids and their derivatives such as succin acid but-3-yn-2-yl 2-methylpent-3-yl ester and heptanoic acid 2-ethyl were accumulated in Meixiangzhan 2. Additionally, 2,2-dichloroethanol, and 2-(octyloxy)ethanol, could be metabolites of dichloroiphenyltrichloroethane (DDT) [25]. Daohuaxiang 2 is planted in Wuchang, Heilongjiang province, which is the most famous aromatic rice-growing area in China [4]. The soil in this region is mainly sandy loam and meadow soil, with abundant sunshine and widespread irrigation systems [6]. The 2-AP content of Daohuaxiang 2 was much greater than the 2-AP contents of the other aromatic rice varieties (Table 1). Higher soil nitrogen levels increase 1-proline content, which is the precursor of 2-AP; thus, aromatic rice from this region has a strong aroma [10; 26]. Guangxi province has a warm climate, abundant rainfall, and moderate sunshine [27].