Chemical composition of essential oils from leaves of Rhododendron dauricum and R. aureum

Abstract

Rhododendron dauricum L. and R. aureum Georgi. (Ericaceae) are medicinal plants used in Tibetan medicine to treat several pathological conditions [1]. Phenolic and terpenoid compounds have been observed previously in these plants [2]. In addition, the compositions of essential oils of both species growing in Russia (Khabarovsk Territory [3], Amur [4], Irkutsk Oblast, Buryatiya [5]) and in China [6] have been reported. Our goal was to study the dynamics of essential oil (EO) composition change of leaves of R. dauricum and R. aureum collected in the vicinity of Lake Baikal during June-August 2008 and in Irkutsk Oblast regions (R. dauricum, experimental plot of SIPFB, Irkutsk; R. aureum, Utulik, Slyudyansk Region). The species were identified by Candidate of Biological Sciences T. A. Penzina [Herbarium samples of raw material (leaves) are preserved in the SIPFB, No. RD128/08-RD130/08, RA064/08-RA066/08). EO was obtained from fresh raw material (storage time <5-6 h) by steam distillation in a Clevenger apparatus for 5 h. Extraction of the distillate with hexane produced an additional amount of EO that was combined with the main portion. EO was studied by GC—MS (Agilent Technologies with mass-selective detector, HP-Innowax column, 30 m/250 μm/0.50 μm, 50-250°C temperature gradient, heating rate 2°/min, He carrier gas, flow rate 1 mL/min). Table lists the analytical results for EO from R. dauricum and R. aureum. About 30 components were observed in EO from R. dauricum. Among these trans-caryophyllene (June, August) and γ-cadinene (July) dominated. The high variability of EO composition during vegetation of the species is noteworthy. Thus, whereas the June samples were dominated by trans-caryophyllene (19.10%), γ-cadinene (13.95%), β-gurjunene (10.46%), and humulene (10.20%); the July and August samples had only traces of β-gurjunene and humulene and high amounts of α-amorphene (8.59-10.00%). The EO content was highest (0.13%) in August. According to the literature, the principal components of R. dauricum EO were limonene, α-pinene, and p-cymene [4]; α-pinene, tetradecane, and 1,2-benzenedicarboxylic acid [5]; and caryophyllene, humulene, and caryophyllene oxide [6]. Studies of EO from R. aureum leaves found that the dominant component of the volatile terpenoid complex was the tricyclic aristolane sesquiterpene calarene, the content of which reached 16-48% (of the EO mass). The content and composition of EO varied during the vegetative period. Its content increased toward autumn (0.18%). The components also changed. The concentration of aristolane derivatives (calarene and aristolene) decreased whereas those of burbonanes (β-burbonene) and cadinanes (α-cadinol and β-cadinene) increased. The Baikal population of R. aureum studied by us differed from that away from Lake Baikal, the principal EO components of which were hexanoic acid, carvacrol, and α-pinene [5].