1.    Introduction

Picea sitchensis (Bong.) Carrière, Pinaceae (Sitka spruce) is a large tree (up to 80 m tall), with drooping branches (Fig. 1A) [1]. The leaves are needles (15-25 mm long), abaxial surface (blue-green), adaxial surface (glaucous) with two dense bands of stomata (Fig. 1B). The bark is thin, scaley, and grayish-brown in color (Fig. 1C). The cones are slender and cylindrical (6-10 cm long) with thin, diamond-shaped scales (12-16 mm) (Fig. 1D). Sitka spruce ranges along a narrow strip along the northern Pacific coast from south-central Alaska south to northern California (Fig. 2) [2, 3]. The tree was introduced to western Europe (Great Britain, France, Norway, Denmark, Germany, Sweden, and Iceland) in the 19^th century [4, 5]. It has also been introduced to New Zealand and Australia [6].

Figure 1. Picea sitchensis (Sitka spruce) in the Oregon Coast Range. A: Branches. B: Leaves (needles). C: Bark. D: Cones.

Figure 2. The native range of Picea sitchensis (Sitka spruce) along the west coast of North America [3].

Picea species have been important sources of traditional medicines throughout their ranges (see, for example, [7–13]). The foliage of P. sitchensis has been used by Native American tribes as a cold medicine (Kwakiutl), as an antirheumatic (Gitksan), and as a gastrointestinal aid (Southern Carrier, Hanaksiala) [14]. Picea species, including Picea abies (L.) H. Karst (Norway spruce), Picea engelmannii Engelm. (Engelmann spruce), Picea glauca (Moench) Voss (white spruce), Picea mariana Britton, Sterns & Poggenb. (black spruce), Picea pungens Engelm. (blue spruce), Picea rubens Sarg. (red spruce), and P. sitchensis (Sitka spruce), are important sources of essential oils [15]. As part of our continuing investigation of essential oils of North American gymnosperms, we have obtained the foliar essential oils from three individual P. sitchensis trees growing in the Van Duzer Forest, Oregon Coast Range. There has been a previous report on P. sitchensis essential oils from coastal British Columbia [16], as well as a report on volatiles identified in P. sitchensis bark extracts [17].

2.    Materials and methods

2.1. Plant Material

Foliage (leaves and twigs) of P. sitchensis were collected using plant pruning shears from the ends of branches from several different positions on three individual trees (samples #1, #2, #3) located in the Van Duzer Forest, Oregon Coast Range on 14 April 2023. The trees were identified by W.N. Setzer based on botanical descriptions [18] and comparison with herbarium samples from the New York Botanical Garden [19]. A voucher specimen (WNS-Ps-6893) has been deposited in the University of Alabama in Huntsville herbarium. The fresh foliage from each tree was combined and the samples were stored under refrigeration (–20 °C). The fresh/frozen samples were hydrodistilled using a Likens-Nickerson apparatus with continuous extraction of the distillate with dichloromethane for four hours to give pale-yellow essential oils (Table 1).

Table 1. Collection and hydrodistillation details of Picea sitchensis from the Oregon Coast Range.

2.2. Gas Chromatographic Analysis

The P. sitchensis foliar essential oils were analyzed by GC-MS, GC-FID, and chiral GC-MS as previously described [20]. The essential oil compositions were determined by comparing both MS fragmentation and RI values with those reported in the Adams [21], FFNSC3 [22], NIST20 [23], and Satyal [24] databases. The percent compositions were determined from raw peak areas (GC-FID) without standardization. Enantiomeric distributions were determined by comparison of RI values with authentic samples (Sigma-Aldrich, Milwaukee, WI, USA), which are compiled in our own in-house database.

3.    Results and discussion

Hydrodistillation of the leaves (needles) and twigs of P. sitchensis gave pale-yellow essential oils in yields of 1.302-2.101%. The gas chromatographic analysis allowed for the identification of 140 chemical components, which accounted for 99.3-99.6% of the compositions (Table 2).

Table 2. Chemical compositions (%) of Picea sitchensis foliar essential oils from the Oregon Coast Range.

 RI_calc = Retention index calculated with respect to a homologous series of n-alkanes on a ZB-5ms column. RI_db = Retention index from the available databases [21–24]  unless otherwise indicated. tr = trace (< 0.05%). ^a The identification is only tentative; although there is a good MS match, the RI values are very different. ^b RI value from Shpatov et al., 2017 [38]. ^c The identification is only tentative; although there is a good MS match, the RI values are very different; however, the compound was identified in the bark of P. sitchensis [17].

The foliar essential oil compositions of P. sitchensis from Oregon are qualitatively similar to those from British Columbia [16]. That is, the major components in both collections were α-pinene, β-pinene, myrcene, β-phellandrene, isoamyl isovalerate, 3-methyl-3-butenyl isovalerate, and piperitone. Furthermore, the major monoterpenoids, α-pinene, β-pinene, δ-3-carene, myrcene, and β-phellandrene, were also observed to be major components in the bark extracts from Vancouver Island, British Columbia [17]. A comparison of the main foliar essential oil components is summarized in Table 3. α-Pinene is a relatively abundant constituent in the foliar essential oils of Picea species [25–27]. On the other hand, β-pinene, myrcene, β-phellandrene, isoamyl isovalerate, 3-methyl-3-butenyl isovalerate, and piperitone were not present in one or more Picea species [25, 28, 29]. Bornyl acetate is often an abundant constituent of Picea essential oils [25, 26], but was not observed in P. engelmannii from northern Arizona [28]. Similarly, camphor is often found in Picea essential oils, but was not detected in samples of P. glauca [25] or P. sitchensis [16] from Canada.

Table 3. Comparison of the percentages of the main components in the essential oils of Picea sitchensis from Oregon, British Columbia, and a commercial sample from New Zealand.

^a Unpublished data from the Aromatic Plant Research Center, Lehi, Utah, USA.

Although the essential oils are qualitatively similar, there are some notable quantitative differences. Monoterpene hydrocarbons were generally higher in the British Columbia samples compared to those from Oregon, while oxygenated monoterpenoid and sesquiterpenoid concentrations were higher in the Oregon samples. It is not clear what factors affect the compositional differences. Previous workers have reported large variations in monoterpene concentrations in essential oils of Picea species both within and between populations [27, 30] as well as seasonal variations in individual monoterpenoid concentrations [31–33]. Furthermore, the monoterpenoid concentrations vary widely between young leaves and older leaves in P. sitchensis; myrcene was found in high concentrations in immature foliage (95%), but decreased with age with concomitant increase in piperitone concentration [34]. The volatile components of Picea species play an important role in avoidance of insects and browsing by herbivores. In P. sitchensis, both myrcene and piperitone affect the feeding behavior of spruce aphids (Cinara costata, Cinara pilicornis, Cinara pruinosa, and Elatobium abietinum) depending on their tolerance to myrcene or piperitone [30]. Total monoterpene concentration was shown to negatively influence browsing of P. sitchensis by red deer (Cervus elaphus) [35]. Genetic, edaphic, climatic, and geographic factors are often cited as affecting the essential oil profiles [36, 37]. Latitudinal differences, including climatic differences, may be responsible for the lower monoterpene hydrocarbon concentrations and higher oxygenated monoterpenoid concentrations in the Oregon samples compared to the British Columbia samples. A commercial sample of P. sitchensis essential oil from New Zealand (unpublished data from the Aromatic Plant Research Center, Lehi, Utah, USA) has also been included in Table 3 for comparison. The concentration of limonene in the New Zealand sample was notably higher, with concomitant lower β-phellandrene, than in the North American samples. The concentration of camphor was also very high in the New Zealand sample, while the total sesquiterpenoids was very low.

The Oregon P. sitchensis foliar essential oils were also analyzed by chiral GC-MS in order to determine the enantiomeric distributions of the terpenoid components (Table 4). The levorotatory enantiomers were dominant for α-pinene, β-pinene, limonene, β-phellandrene, borneol, and piperitone, while (+)-camphor, (+)-δ-3-carene, and (+)-δ-cadinene were dominant. Camphene, linalool, terpinen-4-ol, and α-terpineol were virtually racemic. Robert [17] also found (–)-α-pinene, (–)-β-pinene, (+)-δ-3-carene, and (–)-β-phellandrene to dominate the bark extract of P. sitchensis. However, (+)-limonene rather than (–)-limonene was identified in the bark extract. Consistent with the distributions in P. sitchensis, (–)-α-pinene, (–)-β-pinene, (–)-limonene, and (–)-β-phellandrene were the predominant enantiomers in P. pungens [26] and P. engelmannii [27].

Table 4. Enantiomeric distribution (%) of terpenoid components in Picea sitchensis from the Oregon Coast Range.

RI_db = Retention index from our in-house database developed using commercially available samples on a Restek B-Dex 325 column. RI_calc = Retention index determined with respect to a homologous series of n-alkanes on a Restek B-Dex 325 column. na = reference compound not available. - = compound not observed.

4.    Conclusions

This work presents the first report on the foliar essential oil of Picea sitchensis from the Oregon Coast Range and includes the enantiomeric distributions of chiral terpenoid components. The essential oils of P. sichensis were rich in (–)-α-pinene, (–)-β-pinene, myrcene, (–)-β-phellandrene, isoamyl isovalerate, 3-methyl-3-butenyl isovalerate, and (–)-piperitone. While the compositions are qualitatively similar to those from British Columbia, it would be interesting to compare the Oregon and British Columbia essential oil profiles with populations from Washington state and northern California as well as western Europe in order to more fully appreciate the quantitative differences based on geographical location. Since P. sichensis is commercially important and cultivated for lumber, the foliage recovered represents a value-added commodity, which may be commercially exploited in the essential oil industry. 

Authors’ contributions

Conceptualization, W.N.S.; Methodology, P.S. and W.N.S.; Software, P.S.; Validation, W.N.S., Formal Analysis, P.S., A.P., and W.N.S.; Investigation, P.S., A.P., K.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, P.S., A.P., K.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/). We are grateful to Elizabeth and Dewey Ankney for their assistance in plant collection.

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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