3. Results and Discussion
The leaves from two different mature trees were collected in 2008 and from two different trees in 2009. The leaves were hydrodistilled to give pale yellow essential oils (0.41%–0.47% yield), which were analyzed by gas chromatography—mass spectrometry (
Table 2). The major components in the leaf oils from 2008 were cyclocolorenone (23.8% and 27.5%), α-selinene (21.2% and 15.9%), β-selinene (8.7% and 14.8%), and limonene (8.3% and 8.1%). The 2008 leaf oils were dominated by sesquiterpenoids (79.8% and 82.3%). Although qualitatively similar, there were some important differences in the leaf oils from 2009: Sesquiterpenoid concentrations were lower (53.6% and 68.9%) with correspondingly increased monoterpenoids (45.9% and 32.3%) in 2009; neither α-selinene nor β-selinene were detected in the 2009 leaf oils; there were large concentrations of
trans-muurola-4(14),5-diene (9.5% and 16.8%), which were not detected in 2008, and large concentrations of β-cadinene (13.3% and 11.1%), which was seen in only one tree from 2008 in small (0.2%) quantity. Other major components in the 2009 leaf oils were cyclocolorenone (14.5% and 15.6%), limonene (21.3% and 12.2%), and α-pinene (6.9% and 8.6%).
Cyclocolorenone has been observed in several species of Asteraceae. The compound was found to be a major component of the essential oils of
Solidago gigantea (Asteraceae) (32.8%) [
12,
13],
Solidago canadensis (Asteraceae) (38%) [
14], as well as
Drimys braziliensis (Winteraceae) (18.2%) [
15]. The compound has also been found in the essential oils of
Acritopappus confertus (Asteraceae) [
16] and
Vernonia brasiliana (Asteraceae) [
17], as well as
Ledum palustre (Ericaceae) [
18] and
Eugenia copacabanensis (Myrtaceae) [
19].
Several Asteraceae species have been shown to be rich in α-selinene, including
Baccharis crispa,
Baccharis milleflora [
20], and
Tridax procumbens [
21]. β-Selinene has been observed in
B. milleflora [
20] and
Encelia farinosa [
22]. The leaf oils of
Heterothalamus alienus (Asteraceae) from Argentina [
23],
Tagetes minuta (Asteraceae) from Kenya [
24],
Blumea perrottetiana (Asteraceae) from Nigeria [
25], and
Clibadium leiocarpum from Costa Rica [
26] have all shown
trans-muurola-4(15),5-diene in their compositions.
Comparison of the leaf oils from
M. guatemalensis with those from other
Montanoa species shows little similarity in composition. The major components in the leaf oil from
M. grandiflora from Mexico were α-pinene (4.3%), β-phellandrene (4.2%), limonene (2.4%), citronellal (2.8%), and guayacol (2.0%) [
27], while
M. tomentosa leaf oil from Mexico was dominated by bornyl acetate (26.3%), (
E)-caryophyllene (12.5%), β-cubebene (24.0%), limonene (4.9%), and borneol (4.1%) [
28]. Headspace analysis of
M. tomentosa leaves showed a predominance of monoterpenes, α-pinene (15.9%), α-thujene (10.4%), santolina triene (4.6%), sabinene (39.5%), limonene (3.7%), and γ-terpinene (5.1%) [
29], but volatiles from the glandular trichomes of
M. tomentosa were dominated by the sesquiterpenoids valencene (25.3%–45.0%) and β-eudesmol (27.2%–56.1%) [
30].
The leaf oils of
M. guatemalensis were screened for antibacterial activity against
Staphylococcus aureus,
Bacillus cereus, and
Escherichia coli, but showed only marginal activity against
B. cereus (
Table 3). The oils were also screened for
in vitro cytotoxic activity against MDA-MB-231 human mammary adenocarcinoma and Hs578T human mammary ductal carcinoma cells. The oils did show selective cytotoxicity to MDA-MB-231 cells over Hs578T cells (
Table 3). A previous report indicated the leaf essential oil of
Montanoa ovalifolia from Colombia (composition not reported) had shown marginal cytotoxic activity to Vero and HeLa cells, but no activity against HepG2 or Jurkat cells [
31].
4. Conclusions
This is the first analysis of the volatiles from Montanoa guatemalensis. The leaf oil, rich in cyclocolorenone, α-selinene, and β-selinene, showed selective in vitro cytotoxic activity to MDA-MB-231 cells. It is not clear, however, what components are responsible for the cytotoxicity, but the plant may be a good source of cyclocolorenone. Although this work presents preliminary results, it should serve as a template for further experimentation on the leaf oil compositions of M. guatemalensis, seasonal, individual, and year-to-year variations.