Thymus carnosus Boiss.: Effect of Harvesting Period, Collection Site and Type of Plant Material on Essential Oil Composition

Journal of Essential Oil Research: JEOR, Jul/Aug 2005 by Miguel, M Graça, Duarte, João, Figueiredo, A Cristina, Barroso, José G, Pedro, Luis G

Abstract

The essential oils, isolated by hydrodistillation, from the flowers and the remaining aerial parts (leaves plus stems) from three populations of Thymus carnosus Boiss., collected at different harvesting periods and in three collection sites of Portugal (QL, LM and LA) were analyzed by GC and GC/MS. The oil yields from the flowers collected in LA, QL and LM were six, four and one-fold higher, respectively, than the average oil yields from the remaining aerial parts. Monoterpenes were dominant in all oils (>86%), and sesquiterpenes accounted for 1-6% of the total oils. Diterpenes, phenylpropanoids and non-terpenoid compounds were present in all oils (0.1-0.6%, trace amounts and t-0.4%, respectively). Borneol (26-31%) and camphene (9-18%) dominated QL oils and borneol (18-23%), terpinen4-ol (11-19%) and camphene (8-10%) were the main components of LM oils. LA oils showed high variability in the dominant components, cis-sabinene hydrate (14%), terpinen-4-ol (12%) and borneol (11%) being the main components in the flower oil. Terpinen-4-ol (18%) and borneol (18%) dominated the vegetative phase oil, whereas borneol (21%) and terpinen-4-ol (16%) dominated the aerial parts oils collected in May and July, respectively. QL oils can be classified as a borneol/camphene chemotype and one of the LA samples as borneol/cis-sabinene hydrate/terpinen-4-ol chemotype. However, all the other LM and LA samples do not fit within the previously defined chemotypes.

Keyword Index

Thymus carnosus, Lamiaceae, harvesting period, essential oil composition, borneol, terpinen-4-ol, camphene, cis-sabinene hydrate.

Introduction

Thymus carnosus Boiss. known in Portugal as "tomilhodas-praias," is an undershrub that grows up to 15-40cm high with erect to ascending stems that possess terminal and solitary inflorescences. The fasciculate and thick leaves possess a revolute margin, being glabrous in the adaxial and pubescent in the abaxial epidermal surfaces. The ovate bracts are wider than the leaves and have revolute and ciliate borders. The corolla, up to 5 mm long, is whitish. This species is an endemism of the coastal zones of SW Iberian Peninsula, blossoming from May to August, and can be found in littoral dunes and more rarely in sandy cliffs. In Portugal, T. carnosus grows along the littoral of Estremadura, Baixo Alentejo and Algarve, while in Spain this species has only and rarely been found in the coastal zone of Huelva. This species has been considered to need protection (1-3).

According to the first studies carried out by some Portuguese authors (4,5), carvacrol was the only component identified in the oils of Portuguese T. carnosus. Later on, some Spanish authors (6-8) found borneol, camphene, bornyl acetate and terpinen-4-ol as main components of the oils isolated from plants collected in Spain. A few years later, Salgueiro (2,3) identified three different groups of oils in the flowering aerial parts of 11 populations of T. carnosus collected in Portugal: i) borneol/cts-sabinene hydrate/terpinen-4-ol, ii) linalool/borneol/frans-sabinene hydrate, and iii) borneol/camphene chemotypes. More recently, Sefidkon et al. (9) identified thymol, p-cymene and γ-terpinene as main components of the oils obtained from the dried aerial parts of T. carnosus cultivated in the National Botanical Garden of Iran (Tehran). Nevertheless, the authors neither refer the source of plant material, nor the culture conditions or the time in which the plants were maintained in the culture conditions.

In orderto study the effect of the harvesting period, collection site and type of plant material on oil composition we report in this paper, the chemical composition of the oils isolated separately from the flowers and the remaining aerial parts of three populations of T. carnosus, collected at three different collection sites, in the vegetative, beginning and full flowering phases.

Experimental

Plant material: The aerial parts of 12 samples from three collective populations of T. carnosus were collected during the vegetative phase (February 2000), at the beginning of the flowering phase (May 2000) and during the flowering phase (July 2000) at Quinta do Lago (Algarve), Lagoa de Melides (Baixo Alentejo) and Lagoa de Albufeira (Estremadura), Portugal. Voucher specimens have been deposited in the Herbarium of the Institute Botânico da Faculdade de Ciências de Lisboa under no ACSFGM2000/06/J.

Isolation procedure: The oils were isolated separately from fresh flowers and from the remaining aerial parts (leaves plus stems) (30 g) by hydrodistillation, for 4 h, using a Clevenger-type apparatus.

GC: Gas chromatography analyses were performed using a Perkin Elmer 8700 gas chromatograph equipped with two FIDs, a data handling system and a vaporising injector port into which two columns of different polarities were installed: a DB-I fused-silica column (30 m x 0.25 mm, film thickness 0.25 µm J & W Scientific Inc., Rancho Cordova, CA) and a DB-17HT fused-silica column (30 m x 0.25 mm, film thickness 0.15 µm; J & W Scientific Inc.). Oven temperature was programmed, 45°-175°C at 3°C/min, subsequently at 15°C/min up to 300°C, and then held isothermal for 10 min; injector and detector temperatures, 280°C and 290°C, respectively; carrier gas, hydrogen, adjusted to a linear velocity of 30 cm/s. The samples were injected using split sampling technique, ratio 1:50. The percentage composition of the oils was computed by the normalisation method from the GC peak areas, calculated as mean values of two injections from each oil, without using correction factors.