Research Article
Kathy Swor
Kathy Swor
Independent Researcher, 1432 W.
Heartland Dr., Kuna, ID 83634, USA.
Ambika Poudel
Ambika Poudel
Aromatic
Plant Research Center, 230 N 1200 E, Suite 100, Lehi, UT 84043, USA.
Prabodh Satyal
Prabodh Satyal
Aromatic
Plant Research Center, 230 N 1200 E, Suite 100, Lehi, UT 84043, USA.
William N. Setzer*
William N. Setzer*
Corresponding author
Aromatic
Plant Research Center, 230 N 1200 E, Suite 100, Lehi, UT 84043, USA.
And
Department of Chemistry, University of Alabama in Huntsville, Huntsville, AL 35899, USA.
E-mail: setzerw@uah.edu, wsetzer@chemistry.uah.edu; Tel.: +1-256-468-2862
Abstract
Tsuga
mertensiana (mountain hemlock, Pinaceae) is a
species of hemlock that is native to the west coast of North America, from the
Sierra Nevada Mountains of California to the Kenai Peninsula of Alaska. In this
work, foliage samples of T. mertensiana were obtained from four
individual trees located on Mt. Hood, Oregon. The essential oils were obtained
by hydrodistillation and analyzed by gas chromatography, including
enantioselective gas chromatography. The essential oils were dominated by
monoterpene hydrocarbons (68.8-80.1%) and oxygenated monoterpenoids
(6.4-15.3%). The major components were α-pinene (24.5 ± 3.2%, predominantly
(+)-α-pinene), β-phellandrene (16.3 ± 2.1%, predominantly
(+)-β-phellandrene),
limonene (9.8 ±
6.2%, predominantly (–)-limonene),
benzoic acid (6.4 ±
1.5%), β-pinene (5.9 ± 1.0%, predominantly (–)-β-pinene), (–)-α-phellandrene (5.8 ± 1.1%), and bornyl
acetate (5.7 ±
3.5%). This is the first report on the essential oil composition, including
enantiomeric distribution, of T. mertensiana collected from its natural
habitat and complements our understanding of the phytochemistry of the genus.
Chiral analyses of other Tsuga species would further help in chemically
characterizing the genus.
Keywords
Mountain
hemlock, gas chromatography, chiral, enantiomer.
1. Introduction
Tsuga mertensiana (Bong.) Carrière (mountain hemlock) is an Alpine member of the Pinaceae. A coastal population is found in western North America from coastal Alaska and British Columbia, extending south into Sequoia National Park, California. There is also an inland population in the northern Rocky Mountains of British Columbia, south into northern Idaho and northwestern Montana (Fig. 1) [1–3]. The northern populations (Alaska, British Columbia, Washington, Oregon, Idaho, Montana) are generally T. mertensiana subsp. mertensiana, while the Sierra Nevada, California, population is T. mertensiana subsp. grandicona Farjon [4].
Figure 1. Geographical range of Tsuga mertensiana (Bong.) Carrière (mountain hemlock) [3]. This image is in the public domain of the United States because it only contains material that originally came from the United States Geological Survey, an agency of the United States Department of the Interior.
The trees grow up to 55 m tall; the leaves (needles) spread out from the branches in all directions, 7-20 mm long and 1-1.5 mm wide, bearing stomata on both surfaces; the trunk bark is thick, rough, and longitudinally furrowed, dark gray or dark reddish brown [4] (Fig. 2). Several Tsuga species, including Tsuga mertensiana, have been classified as threatened species and sensitive to climate change [5]. As part of our interest in essential oils of Tsuga species [6–8], we have examined the essential oils composition, including enantiomeric distribution of chiral monoterpenoids, of T. mertensiana subsp. mertensiana from the Cascade Range of Oregon. The volatile composition of T. mertensiana (a cultivated specimen, subspecies not indicated) has been reported [9]. The present report, however, presents, for the first time, the essential oil composition of T. mertensiana collected from its native habitat.
Figure 2. Tsuga mertensiana (Bong.) Carrière (mountain hemlock).
A: Leaves. B: Bark. Photographs by K. Swor at the time of collection.
C: Scan of pressed plant material.
2. Materials and methods
2.1 Plant Material
The fresh foliage of T. mertensiana was collected from four different individual trees located on the southern slope of Mt. Hood, Oregon, on 25 June 2024 (Table 1). The trees were identified in the field by W.N. Setzer using a field guide [10] and later verified by comparison with samples from the New York Botanical Garden [11]. A voucher specimen (WNS-Tm-0262) has been deposited into the herbarium at the University of Alabama in Huntsville. The fresh foliage from each tree was frozen (–20 °C) and stored frozen until distillation. The fresh/frozen foliage of each sample was chopped and hydrodistilled for four hours using a Likens-Nickerson apparatus [12–14] with continuous extraction of the distillate with dichloromethane (Table 1).
Table 1. Plant collection and hydrodistillation details for Tsuga mertensiana from Mt. Hood, Oregon.
Sample | Geographical location | Mass foliage (g) | Mass essential oil (g) | Yield (%) |
A | 45°19′18″ N, 121°42′19″ W, 1663 m asl | 94.92 | 4.1026 | 4.322 |
B | 45°19′18″ N, 121°42′19″ W, 1663 m asl | 91.62 | 2.8413 | 3.101 |
C | 45°19′18″ N, 121°42′20″ W, 1664 m asl | 101.39 | 3.8286 | 3.776 |
D | 45°19′18″ N, 121°42′20″ W, 1664 m asl | 135.41 | 4.9372 | 3.646 |
2.2 Gas Chromatographic Analysis
The T. mertensiana foliar essential oils were analyzed by gas chromatography (GC-MS, GC-FID, and chiral GC-MS) as previously reported [15]. Retention indices were calculated using the method of van den Dool and Kratz [16]. Essential oil components were identified by comparison of mass spectral fragmentation patterns and retention indices found in the Adams [17], FFNSC3 [18], NIST20 [19], and Satyal [20] databases.
3. Results and discussion
The foliage was collected from four different trees growing near Mt. Hood, Oregon. The foliage was hydrodistilled to give colorless essential oils in yields of 3.10-4.32%. Analysis of the essential oils resulted in a total of 87 identified components, representing more than 99% of the essential oil compositions (Table 2). Monoterpene hydrocarbons (68.8-80.1%) and oxygenated monoterpenoids (6.4-15.3%) were the most abundant chemical classes in T. mertensiana, consistent with previous reports of Tsuga volatiles [6–9]. The monoterpene hydrocarbons α-pinene (19.9-26.9%), β-phellandrene (13.3-18.0%), limonene (3.4-15.4%), β-pinene (4.6-6.9%), and α-phellandrene (4.8-7.5%) dominated the essential oils. Benzoic acid (4.8-7.9%) and bornyl acetate (0.7-8.8%) were also relatively abundant. The compositions obtained in this work are similar to those obtained by Lagalante and Montgomery, who analyzed foliage from trees cultivated at the University of Rhode Island using solid-phase microextraction with gas chromatography-mass spectrometry (SPME/GC-MS) [9]. In this previous work, the major components were α-pinene (26.6%), β-phellandrene (19.9%), α-phellandrene (7.3%), β-pinene (7.0%), and germacrene D (21.7%). Limonene concentration was lower (0.9%), while benzoic acid was not observed. In the present study, germacrene D was relatively low in concentration (0.5-2.9%).
Compounds | RIcalc | RIdb | Tree samples | |||
A | B | C | D | |||
(3Z)-Hexenal | 799 | 797 | 0.1 | tr | 0.1 | 0.1 |
Hexanal | 801 | 801 | tr | tr | tr | tr |
(2E)-Hexenal | 849 | 849 | 1.3 | 0.8 | 1.4 | 1.0 |
(3Z)- Hexenol | 851 | 853 | 0.3 | 0.3 | 0.3 | 0.1 |
Tiglic acid | 919 | 911 | 0.1 | 0.1 | 0.1 | 0.1 |
Tricyclene | 922 | 923 | 2.4 | 1.7 | 2.4 | 0.2 |
α-Thujene | 925 | 925 | 0.1 | 0.1 | tr | tr |
α-Pinene | 932 | 932 | 19.9 | 26.9 | 26.5 | 24.9 |
α-Fenchene | 947 | 948 | 0.1 | 0.1 | 0.1 | 0.1 |
Camphene | 949 | 950 | 2.2 | 1.6 | 1.9 | 0.3 |
Thuja-2,4(10)diene | 952 | 953 | tr | tr | tr | tr |
Benzaldehyde | 961 | 960 | tr | tr | tr | tr |
Sabinene | 971 | 971 | 0.3 | 0.2 | 0.3 | 0.1 |
β-Pinene | 977 | 978 | 4.6 | 6.5 | 5.6 | 6.9 |
Myrcene | 988 | 989 | 2.4 | 2.5 | 2.2 | 2.7 |
α-Phellandrene | 1007 | 1007 | 5.3 | 5.8 | 4.8 | 7.5 |
δ-3-Carene | 1009 | 1009 | 0.1 | 0.1 | 0.1 | 0.1 |
α-Terpinene | 1017 | 1017 | 0.4 | 0.2 | 0.3 | 0.3 |
p-Cymene | 1024 | 1025 | 0.7 | 2.7 | 1.4 | 1.1 |
Limonene | 1030 | 1030 | 14.9 | 3.4 | 5.6 | 15.4 |
β-Phellandrene | 1031 | 1031 | 13.3 | 18.0 | 16.2 | 17.6 |
(Z)-β-Ocimene | 1035 | 1034 | 0.7 | 1.3 | 1.5 | 1.8 |
(E)-β-Ocimene | 1045 | 1045 | 0.7 | 0.9 | 0.2 | 0.6 |
γ-Terpinene | 1057 | 1057 | 0.3 | 0.2 | 0.2 | 0.1 |
Pinol | 1071 | 1072 | 0.1 | tr | 0.1 | tr |
Terpinolene | 1085 | 1086 | 0.6 | 0.5 | 0.5 | 0.5 |
p-Cymenene | 1089 | 1091 | 0.1 | 0.1 | tr | 0.1 |
α-Pinene oxide | 1099 | 1097 | - | 0.1 | 0.1 | tr |
Linalool | 1099 | 1101 | tr | - | - | tr |
(E)-4,8-Dimethylnona-1,3,7-triene | 1112 | 1113 | tr | tr | tr | tr |
cis-p-Menth-2-en-1-ol | 1124 | 1124 | 0.5 | 0.9 | 0.5 | 0.7 |
α-Campholenal | 1126 | 1126 | 0.1 | 0.2 | 0.1 | 0.1 |
(4E,6Z)-allo-Ocimene | 1127 | 1127 | tr | tr | 0.1 | 0.1 |
trans-p-Menth-2-en-1-ol | 1142 | 1142 | 0.4 | 0.7 | 0.4 | 0.5 |
Camphene hydrate | 1154 | 1155 | 0.1 | 0.1 | 0.1 | tr |
p-Mentha-1,5-dien-8-ol | 1171 | 1171 | 0.1 | tr | 0.1 | 0.1 |
Benzoic acid | 1180 | 1167 | 7.5 | 7.9 | 5.5 | 4.8 |
Terpinen-4-ol | 1180 | 1180 | 0.6 | 0.5 | 0.5 | 0.2 |
Cryptone | 1187 | 1187 | 0.1 | 0.6 | 0.3 | 0.1 |
p-Cymen-8-ol | 1188 | 1188 | 0.1 | - | - | 0.1 |
α-Terpineol | 1196 | 1195 | 2.7 | 3.1 | 2.6 | 2.1 |
Verbenone | 1208 | 1208 | 0.1 | 0.3 | 0.1 | 0.2 |
Thymyl methyl ether | 1229 | 1229 | 0.1 | 0.1 | tr | tr |
Bornyl acetate | 1284 | 1285 | 8.8 | 6.5 | 6.8 | 0.7 |
(E)-Anethole | 1286 | 1288 | - | - | - | 0.1 |
Methyl myrtenate | 1295 | 1296 | - | - | tr | - |
trans-Pinocarvyl acetate | 1295 | 1296 | - | - | - | tr |
Myrtenyl acetate | 1322 | 1322 | 0.5 | 0.1 | 0.4 | 0.5 |
cis-Piperityl acetate | 1335 | 1335 | - | 0.1 | 0.1 | 0.1 |
α-Longipinene | 1351 | 1352 | - | - | 0.1 | 0.2 |
Neryl acetate | 1359 | 1361 | 0.1 | 0.1 | 0.1 | 0.1 |
α-Copaene | 1376 | 1375 | 0.1 | tr | tr | 0.1 |
Geranyl acetate | 1378 | 1378 | 1.0 | 0.7 | 0.8 | 0.9 |
β-Bourbonene | 1384 | 1382 | tr | tr | tr | 0.1 |
trans-β-Elemene | 1390 | 1390 | 0.1 | tr | 0.1 | 0.1 |
Sibirene | 1403 | 1399 | tr | - | - | - |
Longifolene | 1410 | 1411 | - | - | tr | 0.1 |
β-Ylangene | 1419 | 1422 | tr | tr | tr | tr |
(E)-β-Caryophyllene | 1420 | 1424 | 0.1 | 0.1 | 0.3 | 0.1 |
β-Copaene | 1430 | 1430 | tr | tr | tr | 0.1 |
(E)-β-Farnesene | 1453 | 1452 | tr | tr | tr | tr |
α-Humulene | 1456 | 1454 | 0.1 | tr | 0.2 | 0.1 |
γ-Muurolene | 1475 | 1475 | 0.2 | 0.1 | 0.2 | 0.2 |
Germacrene D | 1481 | 1480 | 2.8 | 0.5 | 2.2 | 2.9 |
β-Selinene | 1489 | 1489 | tr | 0.1 | 0.1 | tr |
trans-Muurola-4(14),5-diene | 1492 | 1492 | 0.1 | 0.1 | 0.1 | 0.1 |
α-Selinene | 1496 | 1497 | tr | tr | 0.1 | 0.1 |
α-Muurolene | 1499 | 1497 | 0.1 | 0.1 | 0.3 | 0.2 |
(E,E)-α-Farnesene | 1504 | 1504 | 0.1 | 0.1 | tr | tr |
γ-Cadinene | 1513 | 1512 | 0.2 | 0.2 | 0.4 | 0.3 |
δ-Cadinene | 1518 | 1518 | 0.5 | 0.5 | 1.2 | 0.8 |
trans-Cadina-1,4-diene | 1533 | 1533 | tr | tr | tr | tr |
α-Cadinene | 1537 | 1538 | tr | tr | 0.1 | tr |
Dodecanoic acid | 1560 | 1559 | tr | - | - | - |
(E)-Nerolidol | 1562 | 1561 | 0.1 | 0.2 | 0.2 | 0.1 |
Salvial-4(14)-en-1-one | 1593 | 1593 | tr | tr | tr | tr |
1-epi-Cubenol | 1628 | 1628 | 0.1 | tr | 0.1 | tr |
τ-Cadinol | 1643 | 1643 | 0.1 | 0.2 | 0.5 | 0.2 |
τ-Muurolol | 1645 | 1645 | 0.2 | 0.2 | 0.7 | 0.2 |
α-Muurolol (= δ-Cadinol) | 1648 | 1651 | 0.1 | 0.1 | 0.3 | 0.1 |
α-Cadinol | 1656 | 1655 | 0.5 | 0.6 | 1.9 | 0.7 |
Benzyl benzoate | 1766 | 1769 | 0.1 | 0.1 | 0.1 | tr |
Benzyl salicylate | 1868 | 1869 | 0.2 | 0.1 | 0.2 | tr |
Palmitic acid | 1958 | 1958 | 0.3 | 0.3 | tr | tr |
Manool | 2053 | 2053 | 0.1 | 0.2 | 0.2 | 0.2 |
Palustrinal | 2229 | 2245 | 0.1 | 0.2 | tr | 0.1 |
Dehydroabietal | 2261 | 2266 | tr | 0.1 | tr | tr |
Compound classes |
|
|
|
|
|
|
Monoterpene hydrocarbons |
|
| 68.8 | 72.5 | 69.7 | 80.1 |
Oxygenated monoterpenoids |
|
| 15.3 | 13.9 | 12.9 | 6.4 |
Sesquiterpene hydrocarbons |
|
| 4.2 | 1.9 | 5.4 | 5.3 |
Oxygenated sesquiterpenoids |
|
| 1.0 | 1.0 | 3.5 | 1.2 |
Diterpenoids |
|
| 0.2 | 0.5 | 0.2 | 0.3 |
Benzenoid aromatics |
|
| 7.8 | 8.1 | 5.8 | 4.9 |
Others |
|
| 2.0 | 1.5 | 1.8 | 1.2 |
Total identified |
|
| 99.5 | 99.4 | 99.3 | 99.3 |
The geographical range of T. mertensiana closely matches the range of Tsuga heterophylla Sarg [8]. There are some notable similarities and differences between the foliar essential oils of these two Tsuga species. The major components of T. heterophylla were α-pinene (18.0 ± 4.7%), myrcene (18.3 ± 7.4%), β-phellandrene (13.6 ± 5.5%), β-pinene (10.4 ± 3.2%), and (Z)-β-ocimene (6.0 ± 3.6%) [8]. α-Phellandrene (2.0 ± 2.1%), benzoic acid (2.5 ± 1.1%), and bornyl acetate (0.1 ± 0.1%) concentrations were lower in T. heterophylla compared to T. mertensiana, while myrcene (2.5 ± 0.2%) was lower in T. mertensiana compared to T. heterophylla.
Enantioselective GC-MS was carried out in order to evaluate the enantiomeric distribution of chiral monoterpenoid components (Table 3). The major enantiomers in T. mertensiana essential oil were (+)-α-pinene (66.3 ± 1.7%), (–)-camphene (77.6 ± 13.5%), (–)-sabinene (85.2 ± 10.3%), (–)-β-pinene (81.8 ± 0.9%), (–)-α-phellandrene (100%), (–)-limonene (86.0 ± 9.9%), and (+)-β-phellandrene (91.9 ± 0.5%). Neither terpinen-4-ol nor α-terpineol showed consistent distributions of the enantiomers. The enantiomeric distributions in T. mertensiana show notable differences compared to those of Tsuga heterophylla Sarg. [8]. In T. heterophylla, α-pinene was virtually racemic, (+)-sabinene was the major enantiomer (91.9 ± 16.6%), (+)-α-phellandrene dominated (93.6 ± 3.8%), (–)-β-phellandrene was dominant (80.7 ± 22.5%), and both (–)-terpinen-4-ol (62.5 ± 6.7%) and (–)-α-terpineol (79.1 ± 9.4%) were the major enantiomers. (–)-Camphene (74.3 ± 7.8%), (–)-β-pinene (96.4 ± 2.4%), (–)-limonene (80.3 ± 8.5%), and (+)-β-phellandrene (80.7 ± 22.5%) were the major enantiomers in T. heterophylla, in agreement with the distributions in T. mertensiana.
Table 3. Enantiomeric distribution (percent) of chiral monoterpenoids in the foliar essential oil of Tsuga mertensiana.
Enantiomers | RIcalc | RIdb | Tree samples | |||
A | B | C | D | |||
(–)-α-Pinene | 977 | 976 | 34.8 | 35.1 | 33.7 | 31.3 |
(+)-α-Pinene | 980 | 982 | 65.2 | 64.9 | 66.3 | 68.7 |
(–)-Camphene | 1001 | 998 | 85.5 | 83.1 | 84.5 | 57.4 |
(+)-Camphene | 1005 | 1005 | 14.5 | 16.9 | 15.5 | 42.6 |
(+)-Sabinene | 1021 | 1021 | 22.8 | 4.2 | 24.4 | 7.7 |
(–)-Sabinene | 1029 | 1030 | 77.2 | 95.8 | 75.6 | 92.3 |
(+)-β-Pinene | 1026 | 1027 | 17.7 | 18.3 | 19.4 | 17.2 |
(–)-β-Pinene | 1030 | 1031 | 82.3 | 81.7 | 80.6 | 82.8 |
(–)-α-Phellandrene | 1051 | 1050 | 100.0 | 100.0 | 100.0 | 100.0 |
(+)-α-Phellandrene | nd | 1053 | 0.0 | 0.0 | 0.0 | 0.0 |
(–)-Limonene | 1074 | 1073 | 94.0 | 72.3 | 85.6 | 92.2 |
(+)-Limonene | 1082 | 1081 | 6.0 | 27.7 | 14.4 | 7.8 |
(–)-β-Phellandrene | 1084 | 1083 | 8.7 | 7.4 | 8.2 | 8.2 |
(+)-β-Phellandrene | 1087 | 1089 | 91.3 | 92.6 | 91.8 | 91.8 |
(+)-Terpinen-4-ol | 1295 | 1297 | 36.8 | 43.9 | 36.4 | 60.9 |
(–)-Terpinen-4-ol | 1298 | 1300 | 63.2 | 56.1 | 63.6 | 39.1 |
(–)-α-Terpineol | 1348 | 1347 | 0.0 | 26.8 | 0.0 | 67.8 |
(+)-α-Terpineol | 1356 | 1356 | 100.0 | 73.2 | 100.0 | 32.2 |
4. Conclusions
This work presents, for the first time, the foliar essential oil of Tsuga mertensiana collected from its natural habitat in the Cascade Mountain Range of Oregon. In addition, the enantiomeric distributions of chiral monoterpenoids have been determined. The major components in T. mertensiana essential oils were (+)-α-pinene, (–)-β-pinene, (–)-α-phellandrene, (–)-limonene, (+)-β-phellandrene, benzoic acid, and bornyl acetate. There are quantitative differences in the volatile components of T. mertensiana from this work and those of T. mertensiana cultivated in Rhode Island. In addition, there are notable differences in enantiomeric distributions in T. mertensiana compared to T. heterophylla. In addition to T. mertensiana and T. heterophylla, there are 10 additional Tsuga species. It would be interesting to determine the enantiomeric distributions in these other Tsuga species for comparison.
Authors’ contributions
Conceptualization, W.N.S.; Methodology, A.P., P.S., and W.N.S.; Software, P.S.; Validation, W.N.S., Formal Analysis, A.P., and W.N.S.; Investigation, K.S. A.P., P.S., and W.N.S.; Resources, P.S. and W.N.S.; Data curation, W.N.S.; Writing – original draft preparation, W.N.S.; Writing – review & editing, K.S., A.P., P.S. and W.N.S.; Project administration, W.N.S.
Acknowledgements
This work was carried out as part of the activities of the Aromatic Plant Research Center (APRC, https://aromaticplant.org/).
Funding
This research received no specific grant from any funding agency.
Availability of data and materials
All data will be made available on request according to the journal policy.
Conflicts of interest
The authors declare no conflict of interest.
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Abstract
Tsuga
mertensiana (mountain hemlock, Pinaceae) is a
species of hemlock that is native to the west coast of North America, from the
Sierra Nevada Mountains of California to the Kenai Peninsula of Alaska. In this
work, foliage samples of T. mertensiana were obtained from four
individual trees located on Mt. Hood, Oregon. The essential oils were obtained
by hydrodistillation and analyzed by gas chromatography, including
enantioselective gas chromatography. The essential oils were dominated by
monoterpene hydrocarbons (68.8-80.1%) and oxygenated monoterpenoids
(6.4-15.3%). The major components were α-pinene (24.5 ± 3.2%, predominantly
(+)-α-pinene), β-phellandrene (16.3 ± 2.1%, predominantly
(+)-β-phellandrene),
limonene (9.8 ±
6.2%, predominantly (–)-limonene),
benzoic acid (6.4 ±
1.5%), β-pinene (5.9 ± 1.0%, predominantly (–)-β-pinene), (–)-α-phellandrene (5.8 ± 1.1%), and bornyl
acetate (5.7 ±
3.5%). This is the first report on the essential oil composition, including
enantiomeric distribution, of T. mertensiana collected from its natural
habitat and complements our understanding of the phytochemistry of the genus.
Chiral analyses of other Tsuga species would further help in chemically
characterizing the genus.
Abstract Keywords
Mountain
hemlock, gas chromatography, chiral, enantiomer.

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This work is licensed under the
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License.(CC BY-NC 4.0).