## 2 Results and Discussion
### Multivariate Analyses of Metabolites in the Five Rice Varieties
#### 2.2.2 Variations in Metabolic VOCs between Aromatic and Non-Aromatic Rice
Thus, the significantly differentially abundant metabolites between Yanfeng 47 and the aromatic rice varieties were mainly associated with the degradation of aromatic compounds and the biosynthesis of secondary metabolites. The main metabolites that were differentially abundant between Meixiangzhan 2 and the non-aromatic rice grains were acetoin, (2-methyloctyl)benzene, 2,6,7-trimethyldecane, 2,2-dichloroethanol, 1-ethenylaziridine, _trans_-2-(2-propynyloxy)cyclopentanol and acetic acid butyl ester (Table 2). The main metabolites that were differentially abundant between Daohuaxiang 2 and the non-aromatic rice grains were 2-AP, acetoin, 2-isopropyl-5-methyl-9-methylenebicyclo[4.4.0]dec-1-ene, 2-methylfuran, 2-methyldecane, 2-butyl-2-ocenal, 2-hexenal, and 3-nonen-2-one (Table 2). The main metabolites that were differentially abundant between Yexiangyoulisi and the non-aromatic rice grain were 2-isopropyl-5-methyl-9-methylenebicyclo[4.4.0]dec-1-ene, 2-methylfuran, 2-hexenal, acetic acid butyl ester, 3-butylpyridine-1-oxide, 6-methyl-3-heptanone, and 2-ethyl-2-(hydroxymethyl)- 1,3-propanediol (Table 2). Overall, the metabolites with the highest abundances in non-aromatic rice were 1,3-dimethoxybenzene, 1,2-dimethoxybenzene, and 5-methyl-2-(1-methylethyl)-2-cyclohex-1-one (Table 3). The pathways of 1,3-dimethoxybenzene and 1,2-dimethoxybenzene are related to benzoate degradation and degradation of aromatic compounds [16]. 5-Methyl-2-(1-methylethyl)-2-cyclohex-1-one is an aromatic compounds usually found in herbal essential oils or Chinese medicines [29] and its pathway is related to the degradation of aromatic compounds [16]. The metabolites with the highest abundances in aromatic rice were 2-hexenal, 2-isopropyl-5-methyl-9-methylenebicyclo[4.4.0]dec-1-ene, 2-ethyl-2-(hydroxymethyl)-1,3-propanediol and 5-ethyl-2-decen-4-one. Hexenal was considered as a marker compound to distinguish aromatic rice from non-aromatic rice varieties [3; 30]. In this study, the content of 2-hexenal in three aromatic rice were much greater than the content of 2-hexenal in the non-aromatic rice varieties, moreover the 2-hexenal content of Daohuaxiang 2 was 38 times higher than the 2-hexenal contents of non-aromatic rice varieties (Table 3). (E)- 2-hexenal is produced by the lipoxygenase pathway and plays an important role in plant defense and protects the microbial proliferation in wounded areas [31]. It induces a significant increase in some membrane related fatty acids, including linear and branched fatty acids and unsaturated fatty acids, and releases free fatty acids [31]. In addition, (E)-2-hexenal has significant antibacterial activity against food spoilage and pathogenic microbial species [31]. 2-Isopropyl-5-methyl-9-methylenebicyclo[4.4.0]dec-1-ene is also an aromatic compound often found in plant essential oils and those essential oils possessed stronger repellency activity against pests [32]. 2-Ethyl-2-(hydroxymethyl)-1,3-propanediol and 5-ethyl-2-decen-4-one are produced by the fatty acids pathway and butanoate metabolism, respectively [16]. The metabolites that differed significantly between the aromatic and non-aromatic rice varieties were associated with the degradation of aromatic compounds, protein digestion and absorption, carbohydrate digestion and absorption, and the biosynthesis of secondary metabolites. VOC metabolites accumulated in aromatic rice varieties were formed mainly from carbohydrates (i.e., terpenes and furanones), fatty acids (i.e., aldehydes and alcohols produced by lipoxygenase or \(\alpha\)- and \(\beta\)-oxidases), and amino acids (acids, alcohols, aldehydes, esters, lactones, flavor molecules containing N and S, benzene, and phenylpropane compounds) [11]. These compounds are positively related to aroma traits and nutritional qualities [33]. Moreover, secondary metabolites associated with biosynthesis of secondary metabolites pathway have obvious impacts on rice quality (nutritional and appearance qualities) [33].