## 2 Results and Discussion
### The Nectars' Chemical Composition of the Headspace, Volatiles and Semi-Volatiles
Other monoterepenes were occasionally found in several nectar types (Table 1). Only a few sesquiterpenes, pharnesyl pyrophosphate derivatives, were present with _trans_-caryophyllene (0.0%-2.2%; 0.0%-2.6%) as the major contributor. _cis_-Jasmone (_cis_-3-methyl-2-(2-pentenyl)-cyclopent-2-en-1-one) was found (1.1%-3.6%; 2.8%-7.9%) in all NE. It is produced by the plants by an oxidative degradation of jasmonic acid (formed by lipoxygenase-catalyzed oxygenation of linolenic acid via 18-carbon cyclic fatty acid formed by the action of hydroperoxide cyclase, followed by reduction and \(\beta\)-oxidants), via 1,2-didehydrojasmonic acid [21]. Subsequent protonation of the carbonyl \(O\)-atom of 1,2-didehydrojasmonic acid is assumed to induce a Grob-type fragmentation of the molecule, yielding CO\({}_{2}\) and _cis_-jasmone (Figure 2a). Among benzene derivatives formed by the shikimate biosynthetic pathway, 2-phenylethanol (1.3%-4.1%; 0.6%-2.8%) and benzaldehyde (0.1%-3.6%; 0.0%-0.6%) were abundant with different distributions among NE\({}_{A}\)-NE\({}_{D}\) (Table 1). Benzyl alcohol, phenylacetaldehyde (more abundant in NE _Okitsu_ and _Zorica_) and methyl benzoate were also found. Phenylacetonitrile (0.0%-7.3%; 0.0%-6.3%) is another benzene derivative containing nitrogen. Its formation has been found in several secondary metabolic pathways initiating from phenylalanine in the plants [22]. Phenyalalanine is first converted to (E,Z)-phenylacetaldoxime, which is then transformed to 2-hydroxy-2-phenylacetonitrile, probably via phenylacetonitrile formation as the intermediate (Figure 2b). Figure 1: Tryptophan biosynthetic pathway. NE\({}_{\rm A}\)-NE\({}_{\rm D}\) headspace also contained lower aliphatic compounds up to C\({}_{10}\) (Table 1), most probably derived from fatty acid degradation. These compounds include alcohols (e.g., (Z)-hex-3-en-1-ol, pentan-1-ol or hexan-1-ol), ketones (e.g., heptan-2-one or octan-2-one), acids (acetic and hexanoic), and methyl esters (octanoate, nonanoate and decanoate). Ultrasonic solvent extracts of NE\({}_{\rm A}\)-NE\({}_{\rm D}\) were strikingly different from the headspace composition, containing less volatile compounds (Table 2). The major ones were (Z)-octadec-9-en-1-ol (14.3%-52.3%), octadecan-1-ol (5.3%-26.0%) and hexadecan-1-ol (6.9%-16.5%). The headspace nitrogen-containing compounds were also identified by USE: 1\(H\)-indole (2.7%-10.3%) and methyl anthranilate (0.9%-11.7%). However, new nitrogen compound appeared in the extracts, 1\(H\)-indole derivative--1,3-dihydro-2\(H\)-indol-2-one (0.2%-18.1%). It is interesting to note that the Schiff base of 1,3-dihydro-2\(H\)-indol-2-one and its synthetic derivatives bearing a 1,3-dihydro-2\(H\)-indol-2-one nucleus were reported for antibacterial, antifungal, anti-HIV, and anticonvulsant activities and GAL3 receptor antagonists [23]. Methylanthine-derivative caffeine was found (1.1%-8.9%) in the extracts of NE\({}_{\rm A}\)-NE\({}_{\rm D}\). Xanthine (3,7-dihydro-1\(H\)- purine-2,6-dione) derivatives belong to purine alkaloids. The major pathway of caffeine biosynthesis is xanthosine \(\rightarrow\) 7-methylxanthine \(\rightarrow\) 7-methylxanthine \(\rightarrow\) theobromine \(\rightarrow\) caffeine [24]. The presence of methylxanthine derivatives in the nectar and pollen of _Coffea_, _Camellia_, _Theobroma_, _Herrania_, _Cola_, _Ilex_, _Paullinia_, and _Cittus_ spp. was previously determined by HPLC [18]. The nectar of _C. paradise_, _C. maxima_, and _C. limon_ contained caffeine (60-487 nmol-mL\({}^{-1}\)), theobromine