## 2 Results and Discussion
### Phytochemical Investigations
#### 2.1.2 Identification of Volatile Compounds by SPME-GC/MS
frticulosa_ subsp. frticulosa, the extract of the aerial parts was constituted mainly of sulfides (primarily dimethyl trisulfide and dimethyl tetrasulfide), which accounted for more than 77% of all volatiles. Among the other chemical classes, esters (8.09%), aldehydes (5.73%), and ketones (2.78%) were the most represented, with methyl benzoate, nonanal, and 4-methyl-3-penten-2-one as the main compounds in the three classes, respectively. These five volatiles, namely, dimethyl trisulfide, dimethyl tetrasulfide, methyl benzoate, nonanal, and 4-methyl-3-penten-2-one, accounted for more than 88% of the volatile fraction of the _M. frticulosa_ subsp. frticulosa extract. The extract of the _M. frticulosa_ subsp. _coronopifolia_ aerial parts showed a volatile profile in which ketones (31.54%) and terpen (23.15%) prevailed, although the other classes were also well represented, ranging between 3.17% (acids) and 11.58% (alcohols). The main volatile compounds were 2-isopropyl-5-methyl-3-cyclohex-1-one among terpenoids; hexahydroanesyl acetone, 4-methyl-3-penten-2-one, and 4-methyl-2-pentanone among ketones; 1-octen-3-ol among alcohols; 4-(methylthio)-butanenitrile and 4-methylpentanenitrile among nitriles; 2-methylbutanal among aldehydes; ethyl decanoate among esters; and dimethyl disulfide and octanoic acid among sulfides and acids, respectively. Figure 2 shows the normalized percentage composition, as classes of substances, of the two subspecies of _M. frticulosa_, highlighting considerable differences between them. In plants, several biosynthetic pathways occur, leading to a wide range of volatile organic compounds as secondary metabolites; thus, differences in the volatile profiles of different plants may reflect differences in their metabolic pathways. These differences have been observed in plants of the same species and even in different cultivars of the same subspecies [13]. The volatile compounds identified in the samples of _M. frticulosa_ subsp. _fruticulosa_ and subsp. _coronopifolia_ can be classified into terpenoids, carotenoid derivatives, fatty acid derivatives, amino acid derivatives, and benzenoid compounds. The terpenoids safranal, \(\beta\)-cyclodextrin, isophorone, and geranylacetone derive from the enzymatic degradation of carotenoids; the first two compounds were present in both subspecies, whereas the others only in subsp. _coronopifolia_, in any case at a low percentage. The other terpenoid compounds were detected only in the extract of _M. frticulosa_ subsp. _coronopifolia_, and all of them were oxygenated monoterpenes; these compounds have not been reported previously among volatiles of _Matthiola_ species. Terpenoids in plants originate from two distinct metabolic pathways: the cytosolic mexulonic acid pathway (MVA) and the plastitial methylerythitol phosphate pathway (MEP). Isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP) are the end products in both pathways and are the precursors of a very large number of compounds by numer  
Figure 2: Normalized percentage composition, as classes of substances, of the volatile profile of _M. frticulosa_ subsp. _fruticulosa_ and subsp. _coronopifolia_ extracts. ous enzyme-catalyzed reactions, including cyclization, hydroxylation, dehydrogenation, oxidation and/or reduction, and isomerization. Aldehydes, alcohols, and esters, containing 6 to 10 carbon atoms, originate from unsaturated fatty acids through the lipoxygenase pathway. In particular, linoleic and linolenic acid are the precursors of the so-called green leaf volatiles, namely, C6 aldehydes, alcohols, and their ester. Linoleic acid is responsible also for the formation of 1-octen-3-one and 1-octen-3-ol, whereas nonanal, 1-nonanol, heptanal, and 1-heptanol are formed from oleic and palmitoleic acids. Among fatty acid derivative compounds, nonanal was the main volatile in subsp. _fruticulosa_, whereas 1-octen-3-ol was the most abundant in subsp. _coronopifolia_, highlighting possible differences between the two subspecies in both their fatty acid composition and the set of lipoxygenases.