Characterization of Essential Oils from Different Taxa Belonging to the Genus Teucrium in Sardinia Island, Italy

The genus Teucrium L. (Lamiaceae) is a genus growing in mild climate zones, particularly in the Mediterranean Basin and Central Asia. It is represented by 11 taxa in Sardinia (Italy), living commonly in sunny habitats. In this study, the following eight Sardinian Teucrium taxa were selected, and the essential oils (EOs), obtained by stem distillation, were analyzed by GC–FID and GC–MS: T. capitatum subsp. capitatum, T. chamaedrys subsp. chamaedrys, T. flavum subsp. glaucum, T. marum, T. massiliense, T. scordium subsp. scordioides, T. scorodonia, and T. subspinosum. The comprehensive analyses led to the identification of 87 constituents representing the majority of the volatile compounds. Significant differences, both qualitative and quantitative, were observed between the taxa. Overall, monoterpenes and sesquiterpenes characterized all Teucrium EOs: T. capitatum subsp. capitatum and T. flavum subsp. glaucum revealed the highest content of monoterpene hydrocarbons, while in the other Teucrium taxa sesquiterpene hydrocarbons prevailed. Worthy of note, diterpenes were found only in T. marum and T. subspinosum, whereas T. massiliense was rich in non-terpenic oxygenated compounds. To the best of our knowledge, this is the first comprehensive report on the chemical composition of EOs obtained from Sardinian Teucrium species.


Introduction
The genus Teucrium (germander) includes 300 taxa belonging to the Lamiaceae family, distributed in Europe, North Africa, and temperate areas of Asia, but mainly concentrated in the Mediterranean region [1,2]. It is represented by herbs or shrubs, with tubular or campanulate calyx, 2-lipped or actinomorphic, 5-toothed, the teeth equal or the upper larger; corolla with one 5-lobed lip; tube without a ring of hairs inside, often included in the calyx; and nutlets smooth or reticulate. Many taxa of the genus Teucrium have been used in traditional medicine as stimulants, tonics, and stomach-ache remedies, for their antioxidant, antifungal, antibacterial, antidiabetic, antiseptic, astringent, antispasmodic, anti-inflammatory, antianemic, and cicatrizant properties, and to treat skin diseases [3][4][5][6][7]. Phytochemical studies reported the presence of both volatile and non-volatile compounds. Of the latter, several neo-clerodane diterpenoids were isolated and considered as chemotaxonomic markers of the genus [4,[8][9][10], along with iridoids, sesquiterpenes, triterpenes, abietane diterpenes, flavonoids, and other classes of specialized metabolites [11]. Much scientific research has already been carried out on essential oils (EOs), since 99 Teucrium

Results and Discussion
The aim of our study was to investigate the volatile fraction obtained from eight Sardinian Teucrium taxa (Figure 1), to improve the knowledge on this genus. In fact, even though the Teucrium genus has been largely studied all over the world, especially for the chemical composition of the essential oils [7], to our knowledge, a comprehensive analysis focusing on Sardinian taxa has never been provided. Furthermore, a comparison between our results and literature data was performed. Many Teucrium species exhibited interesting biological properties such as antioxidant [31], anti-inflammatory [32,33], antitumor [32][33][34], and antimicrobial [5,31], justifying the widespread applications in the ethnomedicine of several countries. A comprehensive survey of traditional uses, chemical composition, and biological properties of the essential oils isolated from Teucrium taxa has been recently published [7]. In our study, all samples were collected during the full flowering period (June-July). Even though the harvesting time is species-specific, it has been previously reported that flowering stage is the best time to harvest aromatic plants belonging to the Lamiaceae family [35,36]. It is known that the phenological stage can greatly affect essential oil composition [37]. In fact, it has been observed that the amount of sesquiterpenes, compounds usually dominant in Teucrium species, slightly decreased in the budding stage, counterbalanced by an increase in oxygenated compounds [38]. Furthermore, the biosynthetic pathway for terpenoids is heavily dependent on environmental factors, such as temperature, water availability, soil composition, and altitude [38][39][40][41][42]. Information on local climate conditions, substratum, altitude, type of habitat, bioclimate-isobioclimates, type of vegetation, and vegetation series of collection sites is provided in Table S1. Except for T. chamaedrys subsp. chamaedrys and T. scordium subsp. scordioides, which were collected from the same location, the other taxa were harvested in several sites located at altitudes between 10 m and 803 above sea level.
The phytochemical analyses showed that, except for T. capitatum subsp. capitatum and T. flavum subsp. glaucum, sesquiterpenes hydrocarbons were the main fraction of the EOs, accounting for 45.18-92.0% of the whole samples. Differently, T. capitatum subsp. capitatum and T. flavum subsp. glaucum EOs were dominated by monoterpene hydrocarbons (87.63 and 76.85%, respectively). Interestingly, diterpene hydrocarbons and oxygenated phenylpropanoids were identified only in T. marum and T. subspinosum, in confirmation of their similarity, both morphological and phytochemical. Moreover, T. massiliense was rich in non-terpenic oxygenated compounds (36.02%). Table 1. List of the components identified in essential oils obtained from eight Sardinian Teucrium taxa selected in this study. For each compound, the relative quantitative amount (area %), expressed as the mean of three analytical repetitions ± SD, is reported. Tmar (T. marum), Tsub (T. subspinosum), Tmas (T. massiliense), Tcsc (T. capitatum subsp. capitatum), Tfsg (T. flavum subsp. glaucum), Tsss (T. scordium subsp. scordioides), Tcha (T. chamaedrys subsp. chamaedrys), and Tsco (T. scorodonia).      As shown in Table 1, the main components detected in EO obtained from Teucrium marum are β-bisabolene (23.04%), β-sesquiphellandrene (17.78%), 3E-cembrene A (14.01%), and (E)-caryophyllene (9.08%). This species, known as "cat thyme" or "mint plant", was previously studied revealing a high variability in the EO composition. Samples originated from Balearic Island were characterized by a high percentage of dolichodial, an iridoid identified in both the organic solvent extracts and simultaneous distillationextraction [43,44]. This compound exerts insecticidal activity and may represent a plant defensive agent [45]. It was also detected in emitted volatile fractions from Corsican samples [5,46], as well as in previously analyzed Sardinian samples [31,43], but not in our oil, pointing out that environmental factors strongly influence the chemical composition of EO, as widely reported in literature [47,48]. Indeed, our samples were collected at sea level, whereas Ricci et al. [31] investigated aerial parts harvested at 800 m altitude. Interestingly, in our study T. marum EO evidenced a similar pattern to Teucrium subspinosum, although quantitative differences in the main compounds have been highlighted. These two taxa are often confused and their taxonomic rank and distribution are still open to discussion [28]. Recently, this taxon was described as endemic to south-western Sardinia and Majorca [25]. Our results, as well as previous phytochemical reports analyzing the EOs from T. marum and T. subspinosum gathered in Sardinia and the Balearic Islands [43,44], were not conclusive and they corroborated the remarkable affinity between the two entities. Differently from what was observed in the volatile fraction, the ethanol extract showed significant differences in the phenylpropanoid glycosides content, suggesting these compounds as markers of these taxa [4]. In fact, verbascoside was detected in both species, but arabinosyl-verbascoside appeared not to be present in T. marum. Both species exhibited antioxidant activity in the DPPH test [4].
Teucrium capitatum subsp. capitatum belongs to the Polium section, a complex section with considerable morphological variations between populations. Previously known as T. polium subsp. capitatum, it has been largely studied all over the Mediterranean Basin, evidencing a very high polymorphism and variability in its volatile constituents. According to a recent report [51], our EO consisted essentially of monoterpene hydrocarbons, being rich in limonene (30.35%), α-pinene (29.79%), β-pinene (9.96%), and myrcene (9.63%). To the best of our knowledge, there is only one report on the EO composition of T. capitatum subsp. capitatum originated from Sardinia [13]. It highlighted the existence of two chemotypes: one characterized by limonene/α-pinene/(E)-nerolidol, predominant in plants growing at sea level, and a second rich in limonene/α-pinene/α-trans-bergamotene/humulene epoxide II, found in mountain samples. Our EO, obtained from aerial parts collected in a costal garrigue, lacked in α-trans-bergamotene, humulene epoxide II, and (E)-nerolidol, confirming the very high variability of this taxon, in accordance with several reports [5,14,32,[52][53][54]. Differently from our EO, in which limonene was the main compound (30.35%), all EOs obtained from Corsican T. capitatum subsp. capitatum were characterized by a lower amount of this monoterpene hydrocarbon, ranging from 3.0 to 5.2%. In addition, a greater quantity of α-thujene (5-8.1%) distinguishes Corsican samples [5,52,53]. We did not detect in our EO the germacrene D, a compound abundant in samples originated from Greece [54,55], Balkan Peninsula [56], and Algeria [57]. Finally, samples from Algeria and Portugal were rich in r-cadinol, differently from all the other analyzed EOs [57,58]. T. capitatum subsp. capitatum was recently reported for its antifungal activity, particularly against dermatophytes and Cryptococcus neoformans, and able to inhibit Candida albicans germ tube formation [13]. It also showed anti-inflammatory and antiproliferative effects on lung carcinoma, colorectal adenocarcinoma, and amelanotic melanoma cell lines, even though the mechanism of action has not yet been explained [32].
Teucrium scordium subsp. scordioides is a perennial herb distributed in wet soil and swamps of southern and south-eastern Europe, Asia Minor, and Morocco. With regard to the analysis of volatile fraction, only two reports have been published concerning Serbian and Sicilian populations [64,65]. Our EO was characterized by a high amount of germacrene D (25.05%), followed by δ-cadinene (12.85%), allo-aromadendrene (11.25%), α-cadinol (6.15%), and germacrene D-4-ol (5.95%), showing significant differences with respect to the ones obtained from Sicilian and Serbian samples. In fact, the EO obtained from Sicilian T. scordium subsp. scordioides was characterized by the presence of caryophyllene oxide (25.8%), α-pinene (19.4%), and β-pinene (8.5%), while the Serbian one contained menthofuran as the main component, a compound totally absent in the other accessions.
Teucrium chamaedrys subsp. chamaedrys grows wild in Central Europe, Asia, and North Africa. This plant is common in Corsican and Sardinian islands. It has been largely studied, both for its volatile and non-volatile fractions. Neo-clerodane diterpenoid derivatives, along with phenylpropanoids and steroidal compounds have been isolated from its aerial parts [66][67][68][69]. There have been previous studies in relation to the EO and quantitative differences in the number of major components have been reported. Indeed, whereas samples collected in Corsica and Sardinia and studied by Muselli et al. [70] were characterized by a high amount of (E)-β-caryophyllene, as well as Serbian samples [71], our EO was dominated by germacrene D (41.85%), similarly to the Turkish ones [72,73]. In our EO, (E)-caryophyllene (29.25%), α-humulene (6.98%) and (E)-β-farnesene (6.20%) were also identified in high amounts.

Plant Material and Essential Oil
Flowering aerial parts of each taxon selected were collected randomly from wild populations, from June to July 2015, in different Sardinian localities. Species were botan-ically identified by Dr. Alfredo Maccioni and voucher specimens were deposited at the General Herbarium of the Department of Life and Environmental Sciences, University of Cagliari (Herbarium CAG). The name of collection sites, along with voucher specimens, GPS coordinates, local climate, and environmental conditions are listed in Table S1.
Plant material was dried at 40 • C to constant weight and then subjected to steam distillation in order to obtain EOs, according to the European Pharmacopoeia, 10th edition. EOs were stored at 4 • C in the dark until the chemical analyses.

GC-FID and GC-MS Analysis
Quantitative analyses were performed on an Agilent 7890A GC (3 repetitions of each sample) equipped with a flame ionization detector (FID) and a 30 m × 0.25 mm i.d. with a 0.25 µm stationary film thickness HP-5 capillary column [78]. Quantification of constituents was calculated by integration of GC-FID peak areas without using the response correction factors. Qualitative analyses were carried out in a gas chromatograph (Agilent 6890N) equipped with a 30 m × 0.25 mm i.d. with a 0.25 µm stationary film thickness HP-5 ms capillary column (Agilent J&W), coupled with a mass selective detector with an electron ionization device (EI) and a quadrupole analyzer (Agilent 5973) [78]. The compounds of the samples were identified by comparing mass spectra fragmentation patterns with those of a computer library, and linear retention indices (RI) were based on a series of C 8 -C 26 n-alkanes homologous to those reported in the literature [79].

Conclusions
This study is a comprehensive report on the EO compositions of eight wild Teucrium taxa, with the aim to increase the knowledge of the genus Teucrium in Sardinia (Italy). In fact, along with species largely investigated, such as T. chamaedrys subsp. chamaedrys, T. marum, T. massiliense, and T. capitatum subsp. capitatum, we analyzed, for the first time, the EOs obtained from some species not yet studied in the island (T. scorodonia, T. scordium subsp. scordioides, Teucrium flavum subsp. glaucum, and T. subspinosum). These species, except for T. subspinosum and T. scorodonia, were reported in Sardinian ethnobotany, confirming the wide use of the genus Teucrium in the traditional medicine systems all over the world. The EOs of each taxon were characterized by GC-FID and GC-MS, and the differences with respect to the literature highlighted. Our results showed, according to the available data, a very high variability of this genus, since significant differences, both qualitative and quantitative were found in the volatile fraction. This variability could be to the consequence of genetic, seasonal, and environmental factors, as is widely documented in literature. Overall, 87 constituents were detected and, except for T. chamaedrys subsp. chamaedrys and T. flavum subsp. glaucum, both characterized by a large amount of monoterpene hydrocarbons, sesquiterpenes hydrocarbons were the main fraction in all the other taxa. Worthy of note, diterpene hydrocarbons and oxygenated phenylpropanoids were identified only in T. marum and T. subspinosum, highlighting their similarity, both morphological and phytochemical. Moreover, T. marum was rich in non-terpenic oxygenated compounds.