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Abstract

The essential oil composition of different aerial parts (stems+leaves and flowers) of Asteriscus maritimus has been analysed by gas chromatography which was coupled with mass spectrometry. A total of 43 compounds have been identified from the different fractions studied, representing more than the 90% of the total oil for each one. Both aerial parts shared most of the main compounds although quantitative differences have been detected. The oils extracted from the stems and leaves showed α-pinene (27.5%) and p-cymene (10.0%) as major components. In the flower oils these compounds were also representative, α-pinene (9.0%) and p-cymene (7.3%), although the main ones were β-phellandrene (11.8%), fonenol (10.9%) and α-phellandrene (10.0%). These compounds were representatively found in the other fraction: β-phellandrene (8.38%), α-phellandrene (7.28%) and fonenol (5.6%). The monoterpene fraction was predominant in the stems and leaves oil (54.6%) while the sesquiterpene fraction was the largest one in the flowers oil (55.3%).

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Keywords

Asteriscus maritimus; Compositae, Essential Oil, Seasonal Variation, α-Pinene, P-Cymene, α-Phellandrene, β-Phellandrene, Fonenol

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Introduction

The genus Asteriscus Miller belongs to the Compositae family. This genus and the other two of the same tribe (Ighermia Wiklund and Pallenis Cass.) contain a total of 15 species that grow wildly in a variety of coastal and desert habitats throughout the Mediterranean and Middle East [1,2]. Although most of the species are endemic of the Macaronesian region, in Europe there are two species described: A. aquaticus (L.) Less and A. maritimus (L.) Less. [3]. A. maritumus (=Odontospermum maritimum (L.) Sch.Bip.=Bubonium maritimum (L.) Hill) is a scabird, hispid perennial species with stems rarely having more than 20cm, woody and much-branched, with petiolate leaves from oblong to oblong-spatulate. The outer involucral bracts are around 1cm, coriaceous below, with an obtuse apex, equalling or shorter than ligules. The ligules are deeply 3-toothed and the tubular florets 5-lobed [3,4].

Although Asteriscus is not a very large genus, few studies have been published related to it [5-18]. The study of the molecular systematic of some Macaronesian species exposed that Asteriscus is paraphyletic [5]. On the other hand, extracts of organic solvents from A. imbricatus DC. showed antifungal activity and anticorrosion inhibition [6]. However, most of the studies have been focused only on one species, A. graveolens (Forssk.) Less., describing their flavonoids [7], new derivatives [8,9] and essential oil activities [10-12]. The essential oil of this species was characterized by a high content of 6-oxocyclonerolidol (66.7%) and 6-hydroxycyclonerolidol (8.8%) which exhibited fungicidal properties towards Alternaria sp. [10]. Other report suggests that these compounds could be considered as chemical markers of this genus that characterize the stem and leaf oil, while cis-8-acetoxychrysanthenyl acetate, which has been isolated for the first time, was the major compound of the flower oil [11]. The last paper related to this species agreed with the previous ones, being 6-oxocyclonerolidol (74.9%) and 6-hydroxycyclonerolidol (11.8%) as the major components. Besides, the essential oil exhibited inhibition of mild steel corrosion [12]. We could have only found other report about essential oil composition [13]. The oils extracted from Pallenis spinosa (L.) Cass (=A. spinosus (L.) Chultz Bip) showed germacra-1-(10), 5-dien-3,4-diol (18.4%), α-cadinol (14.1%), 3-acetoxygermacra-1(10), 5-dien-4-ol (13.0%), t-cadinol (8.2%) and δ-cadinene (5.8%) as main constituents [13].

Asteriscus maritimus has also been studied during the previous years from different point of view. Under water and saline stress conditions, this species showed lower biomass, an early reduction in leaf expansion growth and a substantial degree of stomatal regulation [14]. Recently, biological studies have shown that the diversity of arbuscular mycorrhizal fungi in this species improves salt tolerance by increasing efficiency of photosystem II, stomatal conductance and glutathione content and by reducing oxidative damage [15,16]. Finally, two previous studies have shown that the insecticidal, antimicrobial and anti-acetylcholinesterase activities were revealed by the essential oils [17,18]. The essential oil of aerial parts of A. maritumus from Italy showed insecticidal activity. Fourteen compounds were identified out of which one myrtenyl acetate (44.2%), terphenyl (17.5%) and (Z)-β-farnesene (12.9%) were the principal ones [17]. Th e root oil of Asteriscus maritimus growing in Tunisia yielded Sixty six compounds which represented 96.7% of the total oil, characterized by a high proportion of oxygenated compounds (65.0%). The root oil exhibited antifungal activity against Aspergillus flavus, A. niger, Botrytis cinerea and Penicillium sp [18].

The aim of this paper is to contribute the knowledge of the chemical composition of Asteriscus maritimus species along with the chemical compositions of its different aerial parts and comparing our results with those previously reported.

Aerial parts of Asteriscus maritimus were collected from The Regional Park of Calblanque, Murcia (Spain) in 19-III-2005. All the materials belonged to the same population and around 15-20 specimens were collected. A voucher specimen (MACB-94543) has been lodged at the Herbarium of the Faculty of Biology, Complutense University, Madrid, Spain.

The oils from the different aerial parts of A. maritimus were air dried and were isolated by steam distillation with cohobation for 8 h. according to the method recommended in the Spanish Pharmacopoeia. The oils were dried over anhydrous magnesium sulphate and stored at 4°C in the dark. The different distilled fractions yielded a very low amount of pale yellow oil, being 0.17% and 0.42% from stems+leaves and flowers respectively, based on dry weight.

The analytical gas chromatography (GC) was carried out on a Varian 3300 gas chromatograph fitted with a fused methyl silicone DB-1 column (50 m x 0.25 mm, 0.25 μm film thickness). Temperature was programmed from 95°C-240°C at 4°C min^-1. Injection was performed at 250°C in the split mode (1:100). Nitrogen was used as the carrier gas (1.5 mL in^-1). Detection was performed by FID at 300°C. Injection volume for all the samples was 0.1 μL of pure oil.

GC-MS analyses were carried out on an Agilent Technologies 6890 gas chromatograph coupled a Hewlett-Packard 5973 quadrupole mass detector. Injections were carried out in split mode (20:1) at 250°C. Separations were performed using two columns with different stationary phase (i) a SE-30 capillary column (50 m x 0.22 mm, 0.25mm film thickness) temperature programmed from 70°C to 220°C at 4°C min^-1, and (ii) a DB-Wax column (60 m x 0.32 mm, 0.25 mm) programmed from 70°C to 220°C at 3°C min^-1. Helium at 1 mL min^-1 was used as carrier gas. Mass spectra were recorded in electronic impact (EI) mode at 70 eV, scanning the 35-350 m/z range. Interface and source temperature were 280°C and 230°C, respectively. All analyses were performed in duplicate.

Most constituents were tentatively identified by GC by comparison of their retention indices with those of authentic standards available in the author’s laboratory or with retention indices from references [19-23]. Further identification was achieved by GC-MS: the fragmentation patterns of the mass spectra were compared with those stored in the spectrometer data base using the WILEY.L built-in library. Other constituents were either synthesised or identified in oils of known composition. Semiquantitative analysis was carried out directly from peak areas in the GC profile.

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Results and Discussion

According to our results and previously reported ones, Asteriscus does not produce high quantity of essential oils. While P. spinosa showed the lowest yield (0.04%) [13], the other species of the genus studied till date had similar results: A. graveolens showed an yield (0.06%) [12] and A. maritimus from Italy showed the yield of 0.06% [17]. It is worth noting that for P. spinosa and A. maritimus, the yields were calculated on fresh weight. In our study, the oil amount was little bit higher for both studied fractions: 0.17% for stems+leaves oil and 0.42% for flowers. The yield was based on dry weight that could explain the differences between our results and those above mentioned. Besides, as far as we know it is the first time that the aerial parts of this plant are distilled separately. The flowers contained more oil than the stems and leaves. The oil increase during the flowering period could attract the pollinators of this species to facilitate its cross fecundation, as we have previously published for a relative species, Santolonia rosmarinifolia L. ssp. rosmarinifolia [24]. However, more research should be carried out on this species and other of the same genus to check whether the different fractions of the plant have different yields.

The components identified from the different aerial parts of Asteriscus maritimus, their retention indices and their percentage composition are summarised in Table 1 where all the compounds are arranged in order of their elution on the DB-1 column. A total of 43 compounds have been identified from the different fractions studied, representing more than the 90% of the total oil for each one. We have found qualitative differences between the studied fractions although both of them share practically all their principal and representative constituents. The oil from the stems and leaves showed α-pinene (27.5%) and p-cymene (10.0%) as major components, while from oil from the flower fractions showed α-pinene (9.0%) and p-cymene (7.3%), but out of which the main ones were β-phellandrene (11.8%), fonenol (10.9%) and α-phellandrene (10.0%). These compounds were represented in the other fraction although their percentage compositions were lower than 10%: β-phellandrene (8.3%), α-phellandrene (7.2%) and fonenol (5.6%).