The chemical composition of the aerial parts essential oil of Acinos alpinus subsp. nebrodensis (Lamiaceae) growing in Sicily (Italy)

Abstract Acinos is a small genus of economically important plants belonging to Lamiaceae family whose botanical collocation is quite problematic due to the disagreement among the botanists and the presence in literature of several names and synonyms from different sources. In the present study the chemical composition of the essential oil from aerial parts of Acinos alpinus subsp. nebrodensis (Strobl) Brullo & Brullo collected in Central Sicily was analyzed by GC-MS. The result showed the presence of large quantity of sesquiterpene hydrocarbons with germacrene D (37.9%) as the most abundant component followed by (E)-β-caryophyllene (5.1%). Among the oxygenated monoterpenes thymol (8.3%) was the most abundant; good quantity of hexadecanoic acid (6.8%) was also observed. Chemotaxonomic considerations with respect all the other oils of Acinos taxa, studied so far, were carried out. Graphical Abstract


Introduction
Acinos Miller is a small genus of annual and short-lived evergreen perennial woody plants, belonging to Lamiaceae family, distributed in Europe, Mediterranean, central Asia, North Africa and North America. Its name takes origin from the Greek word ajί mo1 'akinos' that means 'small aromatic plant' (Bonnier 1927;Bown 1995). According to The International Organization for Plant Information (IOPI), genus Acinos Miller comprises 11 species, although some botanists do not agree and several names and synonyms from different sources are present in literature. Even being a small genus, Acinos is an important one within the Lamiaceae family from the economic point of view. In fact, several Acinos species, sold as the herbs of 'mint' group, are available in the markets of some Mediterranean countries (Stojanovi c et al. 2009).
Several traditional applications of Acinos species, depending on the area and on the species, have been reported in folk medicine although they are not justified enough through biochemical researches (Kaya et al. 1999;Mulas 2006;K€ ult€ ur 2007;Karousou et al. 2007).
Several investigations have been carried on the essential oils of Acinos taxa (Table  S1) and their composition will be discussed below.
Acinos alpinus subsp. nebrodensis (Strobl) Brullo & Brullo (syn. Thymus alpinus L., Satureja alpina Scheele, Calamintha alpina Lam., Clinopodium alpinum L., Calamintha pseudoacinos, Calamintha nebrodensis) is a native Sicilian taxon with camephitic habitus, of critical taxonomic attribution, as can be deduced from the large variety of genera to which the Sicilian populations have been referred in recent and past literature. Two different taxa have been identified by Pignatti (1982Pignatti ( , 2018  Studies about the chemotaxonomy of these plants could be crucial to help and facilitate a clear partition of the genus and species, and also provide important information about the effects of the geographic isolation of populations, related to the processes of speciation and differentiation, as well as the specific variability in geographical areas with different edaphic and weather conditions. The analysis carried out in the present work concern to plant samples picked up on Monte Sant'Otiero, a quarzerenitic mountainous relief of Madonie (Sicily), Italy.
Consequently, in the frame of our on-going researches on Sicilian plant essential oil (Gagliano Candela et al. 2020;Rigano et al. 2020;Badalamenti et al. 2021;Ilardi et al. 2021) and on chemotaxonomic differences (Bancheva et al. 2021;Catinella et al. 2021) we decided to investigate the composition of the essential oil of Acinos alpinus subsp. nebrodensis, collected in Sicily.

Chemical composition of essential oil
Hydrodistillation of Acinos alpinus subsp. nebrodensis aerial parts gave a pale-yellow oil (Aan). Overall, forty-eight compounds were identified, representing 96.3% of total components, listed in Table S2 according to their retention indices on a HP-5MS column and classified into five classes on the basis of their chemical structures. Sesquiterpene hydrocarbons formed the main class, representing 50.5% of the oil, with germacrene D (37.9%) as the most abundant component. In the same class, (E)b-caryophyllene (5.1%) was also present in good amounts. Oxygenated monoterpenes were the second most abundant class (18.3%), with thymol (8.3%) as the main component of this class; instead, monoterpene hydrocarbons (9.0%) and the oxygenated sesquiterpenes (7.0%) were present in minor amount.
Comparison with the essential oils of other taxa belonging to Acinos genus (Table  S1) allowed us to carry out some interesting comments. In fact, germacrene D, the main compound of our oil, except for A. majoranifolius, A. suaveolens and for the Greek accession of A. alpinus and A. arvensis, is among the main metabolites of all the other oils. Also (E)-b-caryophyllene is quite common in A. alpinus, A. alpinus ssp. meridionalis, A. arvensis, A. graveolens and A. troodi ssp. grandiflorus. It is noteworthy the presence in Aan of thymol occurring only in the Turkish accession of A. alpinus (Kaya et al. 1999a). Among the other metabolites (9.7% in Aan) it is worthy of mention the presence of hexadecanoic acid (6.8%), also found in A. alpinus (Kaya et al. 1999a), A. arvensis, A. rotundifolius, A. suaveolens (Kaya et al. 1999) and A. troodi ssps. (Kaya et al. 1999b). An aspect that emerges, concerning the main compounds present in the different essential oils of Acinos taxa studied and analyzed (Table S1), is the possibility of distinguishing two chemotaxonomic groups: the first one includes all the species rich in germacrene D (A. troodi ssp. grandifloras, A. rotundifolius, A. hungaricus, A. graveolens, A. alpinus, A. alpinus ssp. meridionalis, A. arvensis); the second one, on the other hand, could contain all those Acinos characterized by a large content of pulegone (A. suaveolens, A. majoranifolius). Only one sample, A. arvensis from Greece, is disconnected from these conclusions as it is rich in pulegone and not in germacrene D.

Conclusions
The comparison of the chemical compositions of the essential oils shows that in many cases they can be used as chemotaxonomic markers to support taxonomic decisions related to the establishment of separate taxa. In our study, the gas chromatographic analysis of the essential oil of Acinos alpinus subsp. nebrodensis showed that this is rich in hydrocarbon sesquiterpenes, representing 50.5% of the oil, with germacrene D (37.9%) as the most abundant component. Thymol, an oxygenated monoterpene, was found to be the most abundant compound in this class (8.3%). Monoterpene hydrocarbons and the oxygenated sesquiterpenes were present, on the other hand, in a minor amount (9 and 7%, respectively). An aspect that emerges from the chemotaxonomic analysis of the main compounds present in the various essential oils of Acinos taxa (Table S1), is the distinction of the taxa into two different groups: the first one includes species rich in germacrene D; the second one, in contrast, groups the species with a high content of pulegone.

Disclosure statement
No potential conflict of interest was reported by the author(s).

Funding
This work was supported by grant from MIUR-ITALY PRIN 2017 (Project N. 2017A95NCJ).