## 2 Results
### Differential VOCs Compositions in 12 Brassicaceae Vegetables  
The VOCs contributing to discriminate between the 12 Brassicaceae vegetables were screened using multivariate statistical analysis and identified by comparison to NIST library (11.0). Information of the identified VOCs, including names, CAS no., retention time, retention indices, VIP and \(p\) values is included in Table S1. In total, 44 VOCs were identified, including two alkanes, five alcohols, four aldehydes, seven esters, seven terpen, 10 isothiates, three nitriles, and six sulphur compounds. Five of the identified VOCs were further confirmed by authentic standards (allyl isothiyanat, 3-butenyl isothianate, PEITC, 3-(methylthio) propyl isothianate and dimethyl disulphide). A heatmap (Figure 2) based on the abundance of VOCs was derived to compare their relative levels in the 12 vegetables. Similar VOCs profiles were found in watercress (_Nasturtium officinal_), rocket (_Eruca sativa_), Brussels sprouts (_Brassica oleracea var. gemmifera_), broccoli (_Brassica oleracea var. italic_), kai lan (_Brassica oleracea var. alboglabra_), choy sum (_Brassica rapa var. parachinensis_) and pak choi (_Brassica rapa subsp. Chinensis_), which had relatively higher levels for most of the VOCs detected as compared to other Brassicaceae vegetables studied. Cabbage (_Brassica oleraca var. capitata_), Chinese cabbage (_Brassica rapa subsp. Pekinensis_) and cauliflower (_Brassica oleracea var. botrytis_) showed similar VOCs profiles, with relatively low levels. By contrast, radish (_Raphanus sativus_) and cherry radish (_Raphanus raphanistrum subsp. Sativus_) both had low levels of VOCs, except for several isothiates and sulphur compounds, which were at relatively higher levels as compared to other vegetables. Figure 1: PCA analysis based on VOCs emitted from 12 Brassicaceae vegetables. In addition, the heatmap also demonstrates that among the 12 Brassicaceae vegetables, the identified VOCs in different chemical groups (e.g., alcohols, aldehydes and esters; terpenes; isothicyanates; nitriles; sulphur compounds) showed distinct profiles (Figure 2). The detection of various VOCs from different chemical groups in the 12 vegetables are described below. _Alcohols, aldehydes and esters_. As shown in Figure 2, these VOCs were present in relatively high levels in watercress, rocket, Brussels sprouts, broccoli, kai lan, choy sum, and pak choi, which are usually referred to as green leafy vegetables grown above ground. In contrast, these VOCs were in very low levels in radish and cherry radish, which are root-type vegetables that grown below ground. The levels of alcohols, aldehydes and esters in cabbage, Chinese cabbage and cauliflower were also low, but were relatively higher than root-type vegetables. The C6 aldehydes, alcohols, and their esters are specifically referred to as green leaf volatiles (GLVs) [1]. In this study, the detected GLVs included 1-hexanol, 3-(Z)-hexenol, 2-(E)-Hexenol, 2-(E)-hexenol, 2-(E)-hexenal, 3-hexenal and 2,4-(E,E)-hexenal acetate, 2-hexenyl acetate, hexyl acetate, (4E)-4-hexenyl propionate, (4E)-4-hexenyl butyrate, and (4E)-4-hexenyl hexanoate. _Terpenes_. Six monoterenes (3-carene, p-cymene, camphene, \(\beta\)-pinene, D-limonene and \(\gamma\)-terpinene) were identified with significant variations among the 12 Brassicaceae vegetables (Figure 2). These monoterenes were found to be most abundant in Brussels sprouts, followed by broccoli, whereas the lowest levels were present in radish and cherry radish. _Isothiocyanates_. Ten ITCs were identified in the 12 Brassicaceae vegetables (Figure 2). The highest levels of several ITCs, including 4-methylpentyl ITC, pentyl ITC, hexyl ITC, heptyl ITC, and 3-methylthiopropyl ITC were found in radish. Allyl ITC, 3-butenyl ITC, and 3-methylbutyl ITC were high in cabbage, and PEITC was the highest in watercress. Relatively lower levels of ITCs were found in rocket, broccoli, cabbage, Chinese cabbage and cauliflower. Figure 2: Heatmap of VOCs emitted from 12 Brassicaceae vegetables.