Composition of essential oil from the leaves of shorea siamensis

Shorea is a genus of about 196 species of mainly rainforest trees in the family Dipterocarpaceae. It is the most important timber genus in tropical Asia and species grow in Sri Lanka, to South China, Malaysia and throughout Indonesia. Many species of this genus have localized distributions on one or only a few islands and so are particularly vulnerable to over exploitation [1]. The genus is a well-known source of oligostilbenoids as principal chemical markers [2–4]. Shorea siamensis (Miq.) Kurz locally known as ‘Temak Batu’ is a shortly deciduous tree that can grow up to 30 m in height. It is commonly found in a canopy tree of dry dipterocarp forests, especially on skeletal soils and overlying granite in Southeast Asia such as Myanmar, Thailand, Malaysia, Cambodia, Laos, and Vietnam. The bark is astringent and the decoction is used in the treatment of dysentery. Besides, the tree is a source of red resin which is used for caulking boats [5]. The plant is classified as ‘Least Concern’ in the IUCN Red List of Threatened Species [6]. Considering the availability of Shorea species and our ongoing research for bioactive essential oils from the flora of Malaysia, we describe here the chemical composition of the leaf essential oil of S. siamensis. The leaves of S. siamensis were collected from Langgun Island, Langkawi in August 2023, and identified by Shamsul Khamis. The voucher specimen (PL-03/23) was deposited at the Herbarium of UPSI. The essential oil was obtained by hydrodistillation (4 h) of the fresh leaves (250 g) using a Clevenger-type apparatus. The oil yield was 0.15% based on the fresh weight. The chemical composition of the isolated oil was analyzed by GC-FID and GC-MS under conditions analogous to those published before [7]. For identification of essential oil components, co-injection with the standards was used, together with correspondence of retention indices and mass spectra with respect to those occurring in the literature [8–10]. AChE inhibitory activity of the essential oil was measured by slightly modifying the previous method [11]. Analysis of the leaf oil led to the identification of 28 chemical components, accounting for 95.1% of the total composition, as shown in Table 1. The group components identified were sesquiterpene hydrocarbons (70.8%) and oxygenated sesquiterpenes (24.3%). The major components identified of the leaves oil were β-caryophyllene (25.2%), germacrene D (19.2%), α-cadinol (6.4%), caryophyllene oxide (5.5%), and τ-muurolol (5.2%). The predominance of sesquiterpenes in leaf oil has often been observed in the essential oil of resin of other Shorea species [12]. In a previous study, β-caryophyllene has been reported as the major component of the essential oils extracted from S. acuminata (leaf oil: 13.9%, stem oil 3.4%) [13], S. robusta (leaf oil: 28.27%; heartwood oil: 24.36%; resin oil: 5.08%) [14], and S. roxburghii (flower oil: 6.47–11.74%) [15]. The essential oil indicated moderate AChE (I%: 45.6% ± 0.2) and inhibitory activity at 1,000 mg/mL concentration, compared to galantamine which gave 95.9% ± 0.2 inhibition.