## 3 Results and Discussion
Further, the final model obtained by tuning process pointed out that Component 1, 2 and 3 comprised 25, 19 and 31 volatiles, respectively (Figure 3B), but with a scarce discrimination among the treatments, as highlighted by the sample plots on the first three components (Figure 3C,D). At 8 days of exposure, three components were selected with a balanced error rate around 0.28 and with a molecular signature composed of 16, 16 and 31 VOCs selected on the first three components, respectively (Figure 4A,B). The sample plots on the three components put into evidence a discrimination among the treatments with 60% of total explained variability split up by 32%, 15% and 13% for the first, second and third components, respectively (Figure 4C,D). In particular, plotting the first two components showed that the BIO treatment was sharply separated from the control and COMB ones by the second component (Figure 4C). Conversely, the first and third components scarcely discriminated among the treatments (Figure 4C,D). All 16 VOCs selected on the second component had a negative weight in the linear combination, and a subset of them, namely (E)-2 hexen-1 ol, (2E)-2-hexenyl propionate, (+)-4-carene, \(\alpha\)-copaene, hexyl propionate, \(\beta\)-phellandrene, \(\alpha\)-pinene, terpionolene, \(\beta\)-pinene, E \(\beta\)-caryophyllene, were found to be emitted at comparatively higher rates in BIO plants (Figure 4E). However, a higher emission was observed for dodecane, 2,6,11-trimethyl in CTR plants, the \(\alpha\)-humulene and \(\alpha\)-terpinene in ABIO ones, and \(\gamma\)-elemene and \(\delta\)-cadinene in COMB ones (Figure 4E). The alcohol (E)-2 hexen-1-ol [59], and the two esters (2E)-2-hexenyl propionate [60] and hexyl propionate [61] are green leaf volatiles which are released in response to different stress conditions to aid in plant defense against herbivory and bacterial and fungal pathogens [62]. The (+)-4-carene, \(\alpha\)-copaene, \(\beta\)-phellandrene, \(\alpha\)-pinene, terpionolene, \(\beta\)-pinene and E \(\beta\)-caryophyllene are terpen, a large family of organic compounds mostly involved in the plant defense. For example, the (+)-4-carene, \(\alpha\)-copaene and \(\beta\)-phellandrene are the most abundant VOCs emitted by _Solanum_ spp. in the presence of _Bacterica cockrelli_ herbivory [63], and terpionolene and E \(\beta\)-caryophyllene were mainly produced by tomato leaves infested with _Trialeurodes vaporario-rum_[64]. Further, the \(\alpha\)-humulene was found to be responsible of tomato repellence against _Bemisia tabaci_[65]. The dodecane, 2,6,11-trimethyl, which, on a comparative basis, was found to be highly emitted by the CTR plants of the present study (Figure 4E), could be regarded as a VOCs marker of healthy plant status, since this same evidence was obtained by [PERSON] et al. in olive plants [66]. Interestingly, the \(\alpha\)-terpinene and \(\delta\)-terpinene were found to be mainly discriminant of ABIO treatment (Figure 4E), thus confirming the results obtained in tomato plants (cv. Gan Liang Mao Fen 802 F1) fertilized with low levels of N [65] and in droughtstressed _Thymus vulgaris_ plants [67]. Finally, the present study revealed the VOCs emittedby multi-stressed tomato plants. In particular, the \(\gamma\)-muurolene, \(\delta\)-elemene, \(\delta\)-cadinene, \(\beta\)-elemene, \(\alpha\)-gauiene, z-\(\beta\)-caryophyllene, aromadendrene, \(\gamma\)-elemene, 1,3,7 Nonatriene 4,8 dimethyl (3E)-, methyl salicate, \(\beta\)-cadinene and myrcene were the compounds constituting the VOCs blend emitted by tomato plants when exposed to combined N and drought stress with infestation by _Tuta absoluta_ (Figure 4E,F). It should be noted that the \(\gamma\)-elemene and \(\delta\)-cadinene were present in both sPLD-DA components while the other volatiles were only observed in the Component 1 that is the lesser discriminant (Figure 4E,F). In _Gossypium arboretum_, the \(\delta\)-cadinene is a precursor of the cyclic secondary sesquiterpene aldehydes, including gossypol, that are insecticides [68].