Leaf Oil of Backhousia enata (Myrtaceae)

Journal of Essential Oil Research: JEOR, Jan/Feb 2007 by Brophy, Joseph J, Goldsack, Robert J, Craven, Lyn A, Ford, Andrew J

Abstract

The leaf oil of the newly described species, Backhousia enata A.J. Ford, Craven et J. Holmes was analyzed by a combination of CC and GC/MS. It has been found to contain cc-pinene (14-17%) and β-pinene (36-42%) as principal components. The oil yield was 0.3-0.7% w/w based on dry leaf.

Key Word Index

Backhousia enata, Backhousia oligantha, Backhousia anisata, Backhousia sp., Anetholea anisata, Syzygium anisatum, Myrtaceae, essential oil composition, α-pinene, β-pinene.

Introduction

Backhousia enata A.J. Ford, Craven et J. Holmes is a recently described species (1) that occurs in northeastern Queensland, Australia, where it is endemic to the "Wet Tropics' and is currently confined to the Tully River catchment area to the northwest of Tully, Queensland. It inhabits notophyll vine forest/rainforest on soil derived from rhyolite and basalt. At present there are less than 200 individuals known.

Backhousia enata is a large shrub or tree growing to 5-15 m in height, with a trunk diameter of up to 20 cm dbh, usually with numerous coppice shoots. It has opposite, laminate, ovate to elliptic-obovate leaves, 24-38 x 11-18 mm in size, with moderately dense oil glands that are conspicuous on both sides. Morphologically, B. enata is most closely related to B. myrtifolia Hook, et Harvey, from which it is geographically separated by approximately 1,000 kilometres. Full morphological details are given in (1).

The leaf oils of all the Backhousia Hook, et Harvey species known up to July 1994 have been investigated (2 and references therein). Since then, the species known as Backhousia sp. (Didcot P.I.Forster 12671) (in 2) has been formally named as B. oligantha Bean (3). Moreover, on the basis of DNA, morphological and anatomical studies, B. anisata has been renamed Anetholea anisata Peter C. Wilson (4). Craven and Biffin, however, considered that the distinction between Anetholea Peter G. Wilson and Syzygium Gaertn. was slight and have now transferred this species to Syzygium under the name Syzygium anisatum (Vickery) Craven et Biffin (5). In this short communication, we describe the composition of the leaf oil of B. enata and compare it with that of other members of the broader genus Backhousia.

Experimental

Field sampling: Leaves were obtained from the type (Ford, Sankowsky and Sankowsky 3792) and also a bulk sample from a further four specimens in the adjacent area at Alcock Forest Reserve, rafting access point No. 9, 5.2 km from Tully River camping area, NW of Tully, Qld, approx 17°45'S, 145°34'E. Voucher specimens are lodged at the herbaria BRI, CANB, K, L, MEL, MO, NE, NSW, QRS, SYD (codes according to 6).

Isolation of oils: The leaf oils were isolated by hydrodistillation with cohobation as previously outlined in (7). Analyses of the oils were carried out by gas chromatography and combined gas chromatography/mass spectrometry. The oil yields quoted below are w/w, based on dry material.

Identification of components: Analytical gas chromatography (GC) was carried out on a Shimadzu GC17 gas chromatograph. Either a SCOT column coated with FFAP or a WCOT DB-Wax (60 m x 0.5 mm, film thickness 1 µm) was used. In both cases the column was programmed from 50°-225°C at 3°C/min with He at 3.5 mL/min as carrier gas. GC integrations were performed on a SMAD electronic integrator without the use of correction factors. GC/MS was performed on a VG Quattro mass spectrometer operating at 70 eV ionization energy; the column used was DB-Wax (60 m x 0.32 mm, film thickness 0.25 µm) programmed from 35°-220°C at 3°C/min, with He at 35 cm/s as carrier gas. Compounds were identified by their identical GC retention times to known compounds and by comparison of their mass spectra with either known compounds or published spectra (8-12).

Results and Discussion

Table I lists the compounds identified in the leaf oil of the type specimen of B. enata. The oil yield was 0.3-0.7% w/w, dry leaf. The leaf oil of this species was dominated by monoterpenes, and these compounds accounted for approximately 80% of the oil, the remainder being sesquiterpenes. The principal components were oc-pinene ( 14-17%) and â-pinene (36-42%), with terpinen-4-ol (5-8%) and p-cymene (2-5%) being the next most abundant components. The major sesquiterpene was spathulenol (3-5%), with no other sesquiterpene being present in amounts greater than 1.5%. No aromatic components were detected in the oil of this species.

The leaf oil composition of B. enata bears no similarity to that obtained from B. myrtifolia, its nearest morphological relative. The major components of this latter species, either singly or together, were the aromatic ethers methyl eugenol, (E)-methyl isoeugenol and elemicin (2). The oil of B. enata is most similar to that obtained from B. sciadophora F. Muell. that contains significant amounts of OC-pinene (44-55%), β-pinene (2-8%) and limonene (6-13%), though the high proportion of β-pinene in B. enata is unusual in the Australian Myrtaceae. In B. sciadophlora monoterpenes accounted for greater than 90% of the oil (2). Backhousia enata falls into a group comprised of B. citriodora F. Muell., B. hughesii C.T. White, B. kingii Guymer and B. sciadophora, which contain either mono- or sesquiterpenoid oils. Morphologically, this group is characterized by the possession of a long, slender hypanthium stipe (1), whereas that of B. enata is short and relatively broad (1.0-1.3 × 0.7 mm). A better appreciation of the relationships of the several Backhousia species than is apparent through morphological and/or essential oil chemical considerations is required and analysis of data derived from DNA sequencing and/or RFLP studies may be informative. The oil from the type of B. enata (see Table I) did not differ significantly in composition from the bulk sample, which would appear to indicate that the oil composition of the species is relatively constant. It may be expected, in view of the small size of the few known populations, that the species is relatively homogeneous which would account for the low variance present in the oil.