Essential Oil Composition of Greek (Origanum vulgare ssp. hirtum) and Turkish (O. onites) Oregano: a Tool for Their Distinction

Journal of Essential Oil Research: JEOR, Jul/Aug 2004 by Kokkini, Stella, Karousou, Regina, Hanlidou, Effie, Lanaras, Tom

Abstract

Aiming to find out if any distinction between the Greek (Origanum vulgare ssp. hirtum) and Turkish oregano (O. onites) essential oils exists, the composition of six wild Greek populations of the former and four of the latter taxon were analyzed by means of GC and GC/MS. A PCA analysis showed that Turkish oregano oils had higher amounts of sabinene, myrcene, [gamma]-terpinene, borneol and carvacrol, and Greek oregano oils had higher p-cymene content. The latter were further divided into two subgroups, distinguished respectively by their thymol and carvacrol content. A comparison of our results with the published information showed that p-cymene percentages > 14% and/or thymol > 6% is found only in Greek oregano, while borneol content > 2% is found only in Turkish oregano oils.

Key Word Index

Origanum vulgare ssp. hirtum, Origanum onites, Lamiaceae, Greek oregano, Turkish oregano, essential oil composition, p-cymene, borneol, thymol, carvacrol.

Introduction

The commercial name Oregano' refers to a number of species that produce essential oils with a characteristic smell due to the high concentration of carvacrol (1). Among the most popular oregano spices are the commercially known as Greek oregano [Origanum vulgare L. ssp. hirtum (Link) letswaart] and Turkish oregano (O. onites L.). Origanum vulgare ssp. hirtum (section Origanum) occurs mainly in the Balkan Peninsula, Turkey andCyprus,betweenseaIevelandl,500m(2) (Figure 1). Origanum onites [section Majorana (Miller) Bentham] has a rather narrow distribution, restricted in Greece and in west and south Turkey; it grows at altitudes between sea level and 1,400 m (3) (Figure 1). It is reported that the essential oils of the two taxa have antimicrobial (4,5-6), antioxidant (7-8), antifungal (9), cytotoxic (6), insecticidal (10) and nematicidal activities (11).

A large part of the commercially available oregano still derives from the wild growing O. vulgare ssp. hirtum and O. onites populations of East Mediterranean (12). As they are usually finely fragmented when trading, their botanical source is difficult to be determined. The present work is a comparative study of the essential oils of Greek and Turkish oregano. In an attempt to find out if the oil composition can be used for the distinction of the two oregano taxa, our results are further discussed in relation to the literature data.

Experimental

Plant material: Aerial parts of O, vulgare ssp. hirtum and O. onites plants were collected during late autumn, from three distinct geographic areas of Greece (Figure 1). Origanum vulgare ssp. hirtum: Athos Peninsula: (1) NE of the town of Karyes, 350 m; (2) S of the town of Karyes, 270 m; (3) Town of Karyes, 500 m. Peloponnese: (4) SW of the city of Gythion, s.l. Island of Crete: (5) S of the village Perivolia, 130 m; (6) Village Meskla, 170 m. Origanum onites: Peloponnese: (7) Near the village ToIo, s.l.; (8) Near the village Skala, 100 m. Island of Crete: (9) Near the village Prasies, 330 m; (10) Near the village Eleftherna, 400 m.

The plants were collected during the post-flowering stage. Voucher specimens were deposited in the Herbarium of the Institute of Systematic Botany and Phytogeography, Aristotle University of Thessaloniki (TAU).

Essential oil isolation: The collected plant material was air-dried at room temperature for 10 days. Then it was grossly pulverized and subjected to hydrodistillation for 2 h using a Clevenger-type apparatus. The oil content was estimated on a dried plant material basis.

Oil analyses: The oils, thus isolated, were chromatographed using a Shimadzu GC-14A gas chromatograph equipped with a Supelcowax 10 capillary column (60 m � 0.25 mm). Injector and detector temperatures were 240�C. The carrier gas was helium with a flow rate 20.4 cm/s (0.6 mL/ min). The column temperature program adopted was: 70�C for 10 min, 70�-180�C at 2�C/min, isothermal for 35 min, then from 180�-200�C at 4�C/min, and finally at 200�C for 5 min. Data were recorded on a Shimadzu CR-4AX integrator and analysed using CLASS-UniPac (Shimadzu) software.

GC/M S analyses of the oils were carried out in a Shimadzu GC-MS QP2000 system (quadropolar system with an ionization energy of 70 eV). The same column and running conditions (temperature program and carrier gas) as in the GC analysis were used. The oil components were identified comparing their relative retention times and mass spectra with those of authentic samples, the Wiley Registry of Mass Spectral Data (13) and published data (14-15) using Benchtop/ PBM Version 3.1 (Palisade Corporation) software.

Statistical analysis: Principal component analysis (PCA) (16) was applied to examine the interrelationships between the studied oils using the following variables: sabinene, myrcene, [gamma]-terpinene, p-cymene, borneol, thymol and carvacrol amount in the total essential oil.

Results and Discussion

The oil compositions of the studied O. vulgare ssp. hirtum and O. onites populations are presented in Table I. Thirtyeight compounds were identified, accounting for more than 95% of the total oil. The oils of the two taxa were qualitatively similar; from the compounds identified, only calamenene was not detected in O. unites. On the contrary, a high quantitative oil variation was found, both within and between the two taxa. The major oil component of Greek oregano was either p-cymene (ranging from 17.7-51.3% of the total oil) or thymol (30.3-42.8%) orcarvacrol (57.4-69.6%), while Turkish oregano was always characterized by the preponderance of carvacrol (54.1-71.2%). Apart from the above constituents, high quantitative oil differences between the two taxa were found in the amounts of sabinene, myrcene, [gamma]-terpinene and borneol (difference of their mean values > 1) (Table I).

In order to find out if any ordination of the studied oils exists, a PCA was applied using as variables the amounts of sabinene, myrcene, [gamma]-terpinene, p-cymene, borneol, thymol and carvacrol. In Figure 2, the eigenvalues are presented along the first two components, accounted for the 89.5% of the total variance. The Greek oregano oils were distinguished by higher p-cymene and they were further divided into two subgroups, mainly due to the opposite trend in the participation of thymol and carvacrol. On the other hand, the Turkish oregano oils formed a distinct group, characterized by higher quantities of sabinene, myrcene, [gamma]-terpinene, borneol, and carvacrol.

A literature survey was carried out to confirm that the above compounds could be used for the distinction of the two oregano taxa. In total, the essential oils of 86 O. vulgare ssp. hirtum and 29 O. onites different populations, covering the whole distribution range of both species, have been up-todate studied. The comparison of their oils showed that: