## 3 Results and Discussion
### Volatile Compounds
#### 3.1.4 Watercress  
A total of 42 compounds were identified or tentatively identified in the headspace of watercress, 26 of which are newly reported (Table 2). The headspace profile was dominated by alcohol and ITC compounds: (\(Z\))-3-hexen-1-ol (**31**, 36%), phenethyl ITC (**24**, 30.7%), 1-penten-3-ol (**25**, 8.1%), and (\(Z\))-2-penten-1-ol (**28**, 4.8%; Figure 2). 1-Penten-3-ol is a compound present widely in Brassicales species [77]. It exhibited a high intensity in watercress leaves, producing a sulphurious and cabbage-like aroma. These attributes are often attributed to ITCs and other sulphur-containing compounds; however, our data suggest that some of these characteristics in watercress could be attributed to this alcohol. (\(Z\))-3-Hexen-1-ol by comparison was much higher in relative abundance but produced a medium intensity aroma that was green and radishy (Table 3). Phenethyl ITC (aroma attributes described in Section 3.1.2.) had one of the highest intensity aromas in watercress, along with phenylacetaldehyde (**52**, 0.2%). The latter, similar to phenethyl ITC, is derived from phenylalanine, but occurs in much lower abundance (Table 2, Figure 2). Its aroma was described as honey-sweet (Table 3) and is likely a significant contributor to watercress odour that has previously gone unrecognised. Other compounds contributing high odour intensities despite low relative abundances were 3-pentanone (**66**, 1.9%) and \(\beta\)-ionone (**98**, 0.4%; Figure 2). 3-Pentanone is regularly reported in Brassicales [3] and was described as high intensity, green, grassy, and floral smelling (Table 3, Figure 2). \(\beta\)-Ionone is common to many plant species as a degradation product of carotenoids [78], and it was described as soapy and fusty by assessors, with a high intensity. Several sulphur-containing compounds, aldehydes, alcohols, ketones, not previously identified in watercress were identified. Methyl thiocyanate (**2**, 0.5%) imparted a sulphury and oniony note, with a medium intensity. This compound is known to be a GSL hydrolysis product of GCP (methyl GSL), but as will be discussed in Section 3.2, this compound was not detected in the UPLC-MS/MS analysis. Therefore, we suggest that it is not directly derived from this GSL and may be a degradation product of other GSL hydrolysis products within the tissues and headspace of the tested Brassicaceae. Cyclopentyl-1-thiaeethane (**8**, 0.5%) was detected, and uniquely present in watercress compared with the other three species tested (Table 2). Little is known about this compound in a biological context. It produced a sweaty, sulphury, medium-weak intensity aroma that is tentatively described in this species for the first time (Table 3). Five aldehydes, two alcohol and two ketones, are newly reported for watercress, which produced perceptible odours within the headspace bouquet: 2-pentenal (**41**, 0.5%), hexanal (**43**, 1.2%), 4-heptenal (**45**, 0.5%), heptenal (**46**, 0.1%), and nonanal (**54**, 0.3%). Both heptenal and nonanal exhibited fatty, green aromas of medium intensity (Table 3) and are common to other Brassicaceae species [77]. 1-Octen-3-ol (**34**, 0.1%) produced a medium strength mushroom-like aroma (as described in Section 3.1.3. ), and 2-phenylethylanol (**37**, 2.6%) produced a floral scent. 6-Methyl-5-hepten-2-one (**68**, 0.2%) has been previously observed in 'wild' rocket and described as having a citrus aroma [55; 79]. In this study, it was also identified with this characteristic, but also as floral and perfume-like, in both watercress and'salad' rocket. It produced a medium-strong aroma in watercress. 2,2,6-Trimethylcyclohexanone (**69**, 0.1%) has been variously described as thujonic, menthol-like, and camphorous [80]. Here, it was described by assessors as imparting floral and green odours, with citrus notes. One ester, (\(Z\))-pent-2-en-1-yl acetate (**57**, 0.1%), was only detected in watercress, and it produced a medium-weak aroma.