The genus Nicotiana of the family Solanaceae contains more than 40 species. Nicotiana tabacum L. and Nicotiana rustica L. are the well-known species spread throughout the world (Pandey and Chada 1998). The leaves of these species yield tobacco and are commonly cultivated as crops. However, N. glauca, also called wild tobacco or tree tobacco, is considered the most powerful of the Nicotiana species, due to its potential for exploitation in the therapeutic field. Like all Nicotiana, it was used for many medical treatments (Watt and Breyer-Brandwijk 1962); it is used to treat burns and inflammatory diseases in some countries (Morel et al., 1998). Warmed leaves are applied to the head to relieve headache, on the throat to relieve pain and put in shoes for painful feet (Van Wyk and Gericke 2000). N. glauca is also known as a highly toxic and teratogenic plant (Panter et al., 1999). The recorded death in humans was due to poisoning by accidental ingestion of the leaves of N. glauca (Sims et al., 1999; Manoguerra and Freeman 1982). A considerable amount of work has been carried out on the metabolism of nicotine in Nicotiana species (Leete and Chedekel 1973) and other works have been conducted on the total alkaloid or nicotine content, which was determined by steam distillation and spectrophotometry. Among the alkaloids in Nicotiana, nicotine was the dominant alkaloid in the leaves of 33 species, nornicotine of 24 species, and anabasine of 2 species (N. glauca and N. debneyi); whereas in the roots of Nicotiana, anabasine predominated in 7 species of Nicotiana including N. glauca. According to (Skliar et al., 2000), results show that N. glauca contains 7-dehydrocholesterol, vitamin D3 and its hydroxylated metabolites. Recently, (Tabana et al., 2016) reported the anti neovascularization effect of scopoletin, an active principle extract from N. glauca, and its antitumorigenic activity on human tumors in xenograft models.

But, to our knowledge no information is available on the essential oil (EO) composition of N. glauca. Essential oils of plants and other products from secondary metabolism have been widely used in traditional medicine, food flavoring, pharmaceutical industries, perfumes and cosmetics (Bauer K, Garbe D, Surburg H. 2008; Price 1998; Satil et al., 2003) and some plant secondary metabolites were implied in the protection against herbivore attack (Kaczorowski and Markman 2016). Some biological activities of essential oils have been known for long time (Digrak et al., 1999; Dang et al., 2001; Grassmann et al., 2000). On the other hand the quantitative composition of the essential oils of many aromatic plants is greatly influenced by genotype and agronomic conditions, such as harvesting time, plant age and crop density (Marotti et al., 1996).

The present study performed on N. glauca, which was harvested from the northern region of Tunisia, provides the content and chemical composition of the essential oil analyzed by GC-MS.

To our knowledge, no investigations have been previously performed on the chemical composition of the essential oil from N. glauca. The essential oil isolated from the leaves of N. glauca by hydro distillation (yield < 1 %, on dry weight basis) was analyzed by GC and GC-MS. The content and distribution of EO in the green leaves of N. glauca are summarized in Table 1. A total of 18 compounds representing almost 93.33% of the essential oil were characterized. Seven compounds were detected but not identified (NI) (Figure 1).

The essential oil of N. glauca contains a complex mixture consisting of mainly phenol, saturated, cylic and terpenic compounds (Table 1). It was dominated by eugenol (58.49%) followed by nonadecane, eugenyl acetate and tridecane,3-methyl at 6.38%; 5.57% and 5.19%, respectively. These compounds were considered as major in the EO; while the percentages of dodecane,2,6,11 trimethyl, tetradecane, docosane, tricosane and 1,2-benzene dicarboxilic, dibutyl ester varied between 1 and 2%. The remaining compounds (included limonene and saturated hydrocarbons (SH)) were only present in small percentage (< 1%), with the exception of triacontane, which was found at 2.62%.

Compared to our results, the same number of compounds was found in the essential oils of N. rustica; whereas twenty-seven EO were detected in N. forgetiana (Schlotzhauer et al., 1995). According to Popova et al., (2017) nineteen compounds in the essential oil of the leaves of N. alata were characterized, but different compounds in each species were detected by GC-MS. Our results showed the presence of 10 n-alkanes (saturated hydrocarbons). However, compared to the above authors, only two saturated hydrocarbons (tricosane and pentacosane) were identified in N. rustica and five SH were identified in N. forgetiana. The percentage of the EO composition was very different in the same genus. In fact, the percentages of tricosane in our study on N. glauca and in others on N. rustica and N. forgetiana were 1.33, 0.38, and 8.16%, respectively. In our research, the major compound was eugenol (58.49%). In contrast, phytol (38.82-48.53%) was the dominant compound in N. alata; nicotine (25.92%) and pentacosane (8.35%) were the major compounds in N. rustica and N. forgetiana, respectively.

The same major compounds in the EO can be found in different plants. Eugenol, the major component of the oil of N. glauca, was likewise present in a higher quantity (72-90%) in the EO fraction of Syzygium aromaticum (Bao et al., 2012). This component varied between 3.1 and 21.1% in the essential oil of Ocimum basilicum cultivated in Turkey, and in the same species the percentage of methyl eugenol was detected at a high level (˃ 34%) (Telci et al., 2006). However, the chemical composition of essential oils was specific to each species, regardless of its origin. In fact, the results showed that the EO of N. glauca (Tunisia) has a specific chemical composition compared to N. rustica, N. forgetiana (Oxford) and N. alata (Bulgaria). This information could be used as a fingerprint for this species.

The content and distribution of EO in the green leaves of N. glauca are provided in this work. Eugenol was the dominant EO followed by nonadecane, eugenyl acetate and tridecane,3-methyl. In general, the N. glauca oil was characterized by a high percentage of the saturated hydrocarbons; while heneicosane, limonene and hexadecane were the minor constituents (˂ 0.5% for each compound) in the EO. This plant is therefore considered a potential source of eugenol (58.49%) and due to this high eugenol content, it can be exploited for medicinal purposes and other applications.