Variation in the Main Components of the Essential Oils from the Leaves and Flowers of Portuguese Thymus albicans Over a Single Season

Journal of Essential Oil Research: JEOR, May/Jun 2004 by Miguel, Maria G, Duarte, Fernanda, Ven�ncio, Flor�ncia, Tavares, Regina

Abstract

The comparison of the chemical composition of the leaf and the flower oils isolated from Thymus albicans Hoffm. et Link belonging to the 1,8-cineole chemotype, collected at Gambelas, Algarve, during the flowering phase (June to September) was carried out. The oils isolated from both the fresh leaves and the fresh flowers by hydrodistillation were analyzed and compared by GC and GC/MS. The oil yields from the fresh leaves ranged from 3.1% in June, to 6.7% in July (full bloom), while the oil yields from the fresh flowers ranged from 4.8%; in July, to 7.0% in September. Independent of the harvesting period, the oils isolated were characterized by their richness in 1,8-cineole either from the leaves (59.7-66.0%) or from the flowers (57.0-66.7%). The flower oils contained the highest concentration of [alpha]-pinene, [beta]-pinene and linalool, while the leaf oils were richest in camphor, citronellol, bornyl acetate and geranyl propionate. Quantitative differences were also found in some oxygenated sesquiterpenes according to the developmental flowering stage being one of the most interesting those registered for viridiflorol, ledol and intermedeol. The highest amounts of these components in both the leaf and the flower oils were observed in July, starting then to decrease until September just as the behavior already found for 1,8-cineole in the flower oils.

Key Word Index

Thymus albicans, Lamiaceae, essential oil composition, 1,8-cineole chemotype, flowers, leaves.

Introduction

One of the most interesting and striking features of the Mediterranean plant communities is their abundance of aromatic plants. Several factors can influence the chemical composition of their essential oils, such as species variety or chemotype, climatic, seasonal and geographic conditions, harvest period, and distillation process, etc. (1-4).

Thymus L. (Lamiaceae) is widely distributed in the Iberian Peninsula, constituting a taxonomically complex group of aromatic plants. Thymus albicans Hoff m. et Link is an undershrub up to 80 cm tall, having unequal calyx teeth (the lower up to 1.5 mm) 2.5-3.5 mm long and a whitish corolla. Thymus albicans being an Iberian endemic species, in Portugal is only found in a restricted area of Algarve, near the sea, in sandy soils, generally in open pinewoods. The oil composition of T. albicans has been evaluated and according to the region, some authors could detect three different chemotypes (1,8-cineole, Iinalool and 1,8-cineole/linalool) (2,4).

In this paper, a comparison of the chemical composition of the leaf and the flower oils isolated from T. alhicam collected at Gambelas, Algarve, is carried out. For this purpose, the oils from the leaves and the flowers collected during the flowering phase were investigated.

Experimental

Plant material: Aerial parts of the same chemotype of Thymus albicam were randomly collected in bloom, at Gambelas (Algarve), from June to September 1999.

Isolation procedure: The oils were isolated from both the fresh leaves and the fresh flowers by hydrodistillation, for 4 h, using a Clevenger-type apparatus.

Gas chromatography: The gas Chromatographie analyses were performed using a Hewlett Packard 5890 Series II gas Chromatograph equipped with an FID, a data handling system and an OV-101 fused silica column (30 m � 0.25 mm, film thickness 0.25 �m). Oven temperature was held at 70�C for 5 min and then programmed to 220�C at 2�C/min. Detector and injector temperatures were set at 260�C and 250�C, respectively. The carrier gas was helium and the working flow was 1 mL/min. The percentage composition of the oils was computed from the GC peak areas without using correction factors. The data shown are mean values of two injections.

Gas chromatograph/mass spectrometry: GC/MS analyses were performed using a Perkin Elmer 8320 gas chromatograph, equipped with a DB-5 fused silica column (30 m � 0.25 mm, film thickness 0.25 �m) and interfaced with a Finnigan MAT 800 Ion Trap Detector (ITD; software 4.1). Oven temperature was held at 70�C and programmed to 180�C at 3�C/min. Transfer line temperature, 250�C; ion trap temperature, 220�C; carrier gas helium adjusted to a linear velocity of 30 cm/s; split ratio, 1:100; ionization energy, 70 eV; ionization current, 60 �A; scan range, 30-400 amu; scan time, 1 s. The identity of the components was assigned by comparison of their retention indices, relative to C^sub 8^-C^sub 21^ n-alkanes, and mass spectra with corresponding data of components from reference oils.

Results and Discussion

The minimal and maximal yields of the oils isolated from the fresh leaves of T. albicans were 3.1% (v/w) in June and 6.7% (v/w) in July, respectively, whereas the oil yields obtained from the fresh flowers ranged from 4.8% in July to 7.0% in September. It is noteworthy that from July to September the leaf oil yields decreased, while the respective flower oil yields increased (Table I).

The percentages of the main components of the leaf and the flower oils isolated from T. albicans during the flowering phase are reported in Table II. The constituents are listed in order of their elution from an OV-IOl column. The main component present in both the leaf and the flower oils was the oxygen-containing monoterpene 1,8-cineole, whose concentration ranged from 57.0% (September) to 66.7% (July) in the flower oils, and from 59.7% (July) to 66.0% (September) in the leaf oils. These high percentages of 1,8-cineole along with low amounts of linalool detected in both the leaf and the flower oils are characteristic of the 1,8-cineole chemotype. In all samples analyzed, linalool was even one of the less significant oxygen-containing monoterpene since the concentrations ranged from 0.3% in the leaf oils to 2.8% in the flower oils.