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10 December 2025

Seasonal Variation in Essential Oil Composition and Yield of Rhus aromatica Aiton (Fragrant Sumac) and a Preliminary Comparison with Rhus trilobata

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1
doTERRA International, LLC, 389 S 1300 W, Pleasant Grove, UT 84062, USA
2
Department of Biology, Utah Valley University, Orem, UT 84058, USA
3
Aromatic Plant Research Center, 230 N 1200 E, Suite 100, Lehi, UT 84043, USA
*
Author to whom correspondence should be addressed.
Molecules2025, 30(24), 4722;https://doi.org/10.3390/molecules30244722 
(registering DOI)
This article belongs to the Special Issue Volatile Natural Products: Extraction, Characterization and Application
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Abstract

Rhus is a genus of shrubs that produces strong aromatic essential oils. These oils have numerous applications, including treating urinary infections, flavoring food, and alleviating gastrointestinal ailments. We conducted an experiment to track the essential oil yield and composition of two Rhus species, Rhus aromatica Aiton and Rhus trilobata Nutt., over the course of the growing season. The essential oil yield and composition were examined over four weeks from May to June 2023. We found that higher ambient temperature lowered the total yield of essential oils. Yield decreased in both species over the course of the experiment, with a yield of 0.21% in week 1 and 0.07% in week 4. The composition of oils changed as the outside temperature increased. Some of the lighter hydrocarbons, such as limonene, decreased, while other heavier hydrocarbons, including β-caryophyllene and germacrene D, increased as the outside temperature rose. Limonene was also shown to be the main component of R. aromatica essential oil at an average of 86.2% area, with >98% of that being the enantiomer D-limonene. Additionally, the effect of different drying methods on the yield and composition of the essential oils was investigated. The results show that when drying the plant material in the oven, a greater yield of 0.31% is obtained, whereas the shade- and sun-dry methods yield 0.09% and 0.02%, respectively. The main component of R. trilobata was α-pinene compared to the D-limonene in R. aromatica. This study helps to understand more of the correlation between outside temperature and essential yield/composition in the species R. aromatica, particularly during the May–June sampling period. To compare two Rhus species that get commonly grouped together as the same species, SPME Headspace was used to determine that the main component of R. trilobata was α-pinene compared to D-limonene being the main component in R. aromatica.

1. Introduction

Plant phytochemicals, chemical compounds synthesized by plants, have been well studied for their important ecological roles [1,2] evolutionary context [3,4], and use by humans [5]. The controls of the content, quality, and quantity of these phytochemicals are dictated by multiple inputs and at various scales [6,7]. Within groups of plants, and even within individual species, phytochemicals can vary over both space and time [8,9]. Understanding this diversity of variation in phytochemistry is a major goal for both elucidating the evolutionary ecological context of plants as well as for humans who take advantage of these compounds.
One of the prominent predictors of plant phytochemistry is the phenological patterns that plants follow in synthesizing these compounds [9]. Changes in phytochemicals in plants can fluctuate daily [10,11] as well as change seasonally [12]. As with any phytochemical composition, these phenological patterns can influence the content [13], quality [14], and quantity [15] of the phytochemicals that are produced by a plant. These daily and seasonal fluctuations are typically controlled by patterns such as water availability [16], temperature [17], solar availability [18], nutrient fluctuations [19], and/or biological responses [20].
The mechanisms regulating phytochemicals that are triggered by these biotic and abiotic cues are controlled by changes to genetically based traits (i.e., selection) and through tweaks to biochemical pathways. Between species, selection on biochemical pathways is likely the central point of divergence between related species [21]. For instance, cultivars of hemp and cannabis breeding have taken place over generations to emphasize particular biochemical pathways that have resulted in the production of particular phytochemicals [22]. Within a species phytochemicals vary largely by responses within (not between) biochemical pathways [23]. In Teucrium arduini, variation in phytocompounds can be a response to local environmental conditions [24]; this is likely related to cues that trigger changes in local biochemical pathways [25].
Because the ability to predict phytochemical concentration for specific uses is important from both ethnopharmacological and commercial perspectives, many cultures have developed patterns of plant use that take advantage of this phenology [26]. For example, traditional plants used for their antimicrobial properties in South Africa show distinct seasonal fluctuations in tannin and flavonoid levels [27], while Artemisia dracunculus is known to produce higher quantities of essential oil when water deficient [28]. In Uganda, rural communities will typically collect the important socio-medicinal plant Moringa oleifera only in the dry seasons in order to secure plants that have desired phytochemical levels [29]. In commercial applications of plant phytochemicals, timing and processing of plants are important considerations in the content, quality, and quantity of extractable products [30,31].
The genus Rhus (Anacardiaceae) comprises over 250 species of deciduous trees and shrubs, distributed worldwide. North American indigenous people have used them as dyes [32] and as medicines [33,34,35]. They have also been used medicinally and as a flavoring in Iran [36]. Of the genus, Rhus aromatica Aiton and Rhus trilobata Nutt. Both have had a prominent place in North American herbology [37].
Rhus trilobata and R. aromatica both have trifoliate leaves and are native to North America. R. trilobata is native to the western region of North America and parts of western Canada and Mexico, and R. aromatica occupies areas east of the Great Plains. Rhus trilobata generally has smaller flowers and fruits, as well as more distinctly lobed terminal leaves; however, it is challenging to use these characteristics to differentiate the two species. The best way to distinguish them is by geographical distribution [38]. Herbarium specimens have been collected and submitted to the UVSC Herbarium.
Both plants are used for their essential oils rich in the main component of D-Limonene. D-limonene is a monocyclic monoterpene that is abundantly found in citrus plants, such as lemons, oranges, limes, and grapefruits [39]. Limonene has a pleasant orange-like odor and is a colorless oil [40]. Some of the applications of D-limonene include having antitumor and anticancer properties, as it promotes apoptosis in certain cancer cells [41], as well as antioxidant, antidiabetic, and anti-inflammatory activity [39].
In this study, we investigated the seasonal variation in essential oil composition and yield in R. aromatica. We also wanted to compare this variation and yield to the closely related R. trilobata. Our goal was to identify the most efficient and productive time and post-harvest drying to extract the essential oils of these important ethnomedicinal plants for better future use.

2. Results

2.1. Essential Oil Yields Based on the Time of Year Collected

Rhus aromatica essential oils were shown to decrease in yield as temperature increased in the area where the plant material was collected. At an average temperature of 21 °C, the highest average temperature, we found that essential oil yields were the lowest, at an average of 0.06%. In the coldest average temperature of 15 °C, we found that essential oil yields were the highest at 0.21% (Table 1).
Table 1. The plant material and essential oil mass with the yield of each experiment.
During the first week, we achieved a crude yield of 0.21% (Week 1) and 0.35% for the flowers (Table 1). After the first week, the yields dropped to 0.15% (Week 2) and then decreased to below 0.10% for the remainder of the experiment. This drop was paired with a corresponding increase in temperature over the observed weeks (Table 2).
Table 2. Average temperature of each month the experiment was run.

2.2. Essential Oil Composition Based on the Time of Year Collected

The essential oils from each distillation were subjected to detailed GC-MS, GC-FID, and GC Chiral analyses to determine the compositions of each oil based on the week that it was collected. Lighter hydrocarbons, such as limonene and α-pinene, were shown to decrease as the temperatures rose from May to July in 2023. In contrast, other compounds, such as β-caryophyllene and germacrene D, showed a trending increase from the same months (Table 3).
Table 3. GC-FID results showing essential oil percent composition of R. aromatica. Area % refers to GC-FID data.
Aerial parts (leaves, flowers and stems) were collected individually for two different distillations (neither distillation produced enough oil to run through GC-FID, so the hydrosol was compared as shown in (Table 4).
Table 4. GC-MS results showing percent composition of R. aromatica hydrosols. R. trilobata hydrosol shown for comparison. Area % obtained through GC-MS.

2.3. Hydrosol Composition and Trends

The hydrosol compositions exhibited significant variations; however, certain compounds were consistently identified across the GC-MS reports. Terpinen-4-ol, α-terpineol, epi-α-cadinol, epi-α-muurolol, and α-cadinol are some of the most consistent compounds shown in the hydrosols that were tested. α-Cadinol was the main component of the R. aromatica and R. trilobata hydrosols. Because alcohols are more soluble in water than hydrocarbons, the alcohols shown here are more likely to show up in the hydrosol instead of staying in the essential oils during distillation, which is the reason for the high concentrations of these compounds in the hydrosols (Table 4). Because the hydrosol results varied significantly, it is challenging to identify an exact trend of compounds in relation to seasonal variation or drying methods. Because hydrosols contain the hydrophilic fraction of the distillate, higher levels of water-soluble compounds are expected. Solvent extraction and distillation heat may also introduce variability, which can explain elevated values such as octanoic acid.

2.4. Effect of Drying Method on Essential Oil Yield and Composition

When drying the plant material, the results show that the oven-drying method yielded more essential oil than the sun- or shade-drying methods. When drying the plant material in an oven, we obtained an essential oil yield of 0.31%, whereas the sun- and shade-dried methods yielded 0.09% and 0.02%, respectively (Table 1). In the oven- and shade-drying methods, we observed that the limonene content in the essential oils decreased compared to the freshly distilled material, with an average area of 78.6% and 86.2%, respectively (Table 3).
Each drying method was found to increase the area percentage of sesquiterpenes, including β-caryophyllene and germacrene D, which are heavier hydrocarbons compared to limonene. This may be caused by the high boiling point that sesquiterpenes possess, causing these molecules to increase in concentration as the temperature rises in the plants. We observe that the oven-drying method increases the germacrene D levels to 5.4%, compared to the 4.3% achieved by the shade-drying method (Table 3). Moisture content was not measured for the dried samples, which limits direct comparison with the fresh-weight yields reported for the weekly harvests.

2.5. Chemical Composition of Rhus aromatica Flower

Compared to the rest of the experiments performed on mostly leaf material, the distillation composed entirely of R. aromatica flower was different in that the limonene content was down to around 76.5% area, and the α-pinene content was more than that of the leaves at 6.9% compared to around 1% area (Table 3). It was expected that the flower of the plant would show different chemistry than the leaves and stems of the plant.

2.6. Chemical Difference Between Rhus aromatica and Rhus trilobata Using SPME Headspace

As demonstrated throughout this study, limonene is the primary compound of R. aromatica, as confirmed by SPME headspace analysis. SPME Headspace analysis showed that limonene remains the primary compound of R. aromatica. Because the plants are so similar, and since some taxonomists categorize R. trilobata as a subspecies of R. aromatica, it is assumed that this would also be the case for R. trilobata. However, the reports show that the main component in R. trilobata was α-pinene (22.3% area), and only 2.3% of the area was limonene. Sesquiterpenes such as germacrene D and β-caryophyllene are also a lot higher in R. trilobata compared to R. aromatica (Table 5).
Table 5. SPME headspace to compare Rhus aromatica and Rhus trilobata.
SPME headspace analysis was carried out only on a single representative sampling rather than across all harvest weeks because this experiment was intended as a qualitative, exploratory comparison of volatile profiles, not as part of the seasonal quantitative dataset. GC-MS, GC-FID, and chiral GC were therefore used for all week-to-week comparisons.
With the use of GC Chiral, the enantiomer D-limonene was found to be the main component of the essential oils at >98% of the total limonene content, with <2% being the L-limonene enantiomer (Table 6). This can explain its “lemony scent” and a lot of its medicinal properties as well.
Table 6. GC Chiral results showing the enantiomers of some of the compounds found in R. aromatica—enantiomeric distribution.

3. Discussion

Our results found variation in the phytochemical concentration of R. aromatica throughout the growing season. Although plant biomass increased over the course of our study, the essential oil yield dropped by around 50% (Table 3). This change is not all that unexpected, as many plants contain their highest levels of phytochemical production during times of early development and growth [9]. In addition, as the weeks progressed, we also saw a decline in yield that was associated with increasing average temperatures (Table 2). This could suggest that R. aromatica shifts resources and allocation within its biomass to either buffer itself from the increase in temperature [42] or that during plant phenology, there is a gradual reduction in these phytocompounds. It is important to point out that both ideas are not mutually exclusive [43].
Post-harvest drying was also an important factor in phytochemical extraction (Table 3). Notably, several phytochemicals increased the area percentage of sesquiterpenes. This may be caused by the high boiling point that sesquiterpenes possess, causing these molecules to increase in concentration as the temperature rises in the plants [44]. Post-processing material is a crucial aspect of extractions, particularly for specific compounds of interest. This is demonstrated in the traditional and contemporary harvesting and use of many plants for their compounds [45]. For instance, in Africa, Vernonia amygdalina must be rapidly dried at relatively high temperatures to preserve the most important phytochemical compounds in the plant [46]. Likewise, in our study system, quick drying with an oven instead of a longer process outside (in the sun or shade) would likely best preserve and concentrate the target phytocompounds. This is likely due to the rapid removal of moisture, which prevents the enzymatic breakdown of compounds [47]. Please note that these drying methods were performed in July using material different from that collected in May and June, separating these results from our seasonal variation effects results.
The monoterpene D-Limonene stands out as the primary compound found in Rhus aromatica. When we compare this to common citrus essential oils—specifically those derived from Citrus sinensis and Citrus grandis—limonene content often sits between 63% and 89%. Notably, the (+)-enantiomer in citrus peel oils grown in Nepal can achieve an impressive purity of up to 99.4%, a figure established by Bhandari et al. in 2021 [48].
Interestingly, our Rhus aromatica samples demonstrate a comparably high enantiomeric purity. What sets the Rhus samples apart, however, is their significantly greater stability across different seasons. Citrus oils, conversely, frequently experience fluctuations in both limonene levels and chiral ratios, which are sensitive to variables like altitude, fruit maturity, environmental temperature, and other factors. The consistent maintenance of a high proportion of (+)-limonene in Rhus aromatica strongly suggests a greater influence of genetic control and a much lower sensitivity to environmental variations than what we typically see across commercial citrus chemotypes.
We found significant differences between R. aromatica and R. trilobata, although they are very closely related (and once assumed to be the same species [38]). Phytochemical signatures of plants are typically seen as a great way to distinguish species [49] and infraspecific ranks [50]. These differences often reflect the consequences of differences in selection pressures. For these two allopatric species, selection pressures differ, likely not just from climate alone [51] but also from biotic selections [52]. These changes can happen very rapidly; in the case of Erythranthe guttata, separation of just over a century has caused divergence in phytochemistry between non-native plants and native plants that rivals the differences between genetic clades of native plants [53]. In the case of these two species of Rhus, the continental-scale divergence would easily predict our observed outcomes. Mass, fiber type, extraction time or temperature were not standardized in this experiment comparing the two species using the SPME Headspace.
This work performed on the essential oils of R. aromatica provides new information on its chemical composition, drying methods, and species differentiation. The results have indicated that the essential oil yields and the main component, D-limonene, are more prevalent when the plant is harvested at cooler temperatures in its environment, thus preserving the main compounds and achieving a higher essential oil yield. It is best to harvest the plant for its essential oils earlier in the spring to maximize the essential oil yield and D-limonene content and to use the medicinal properties. Waiting to harvest them until later in the season could risk losing important compounds that give the plant its medicinal properties. This species and its essential oils have potential for medicinal use in the future, and further research is necessary to fully understand their potential.

4. Materials and Methods

4.1. Rhus aromatica and Rhus trilobata Plant Material

Rhus aromatica Aiton was collected every eight days (Week 3 did not produce any oil, so the experiment was re-conducted on 14 May 2023 and replaced “Week 3” results) on doTERRA International, LLC (Limited Liability Company) campus grounds in Pleasant Grove, Utah 84062, USA (40°21′26.3″ N 111°45′27.1″ W, 1373 m elevation), starting on 22 May 2023 in the mornings from 9 to 11 am, and ending on 16 June 2023 on week 4. The aerial parts of the plant, including the flowers, leaves, and some of the stem, were collected. It was collected from various bushes of the same species across the doTERRA campus grounds. 100–300 g of material were collected each time. We pooled these results to show the trends. The flower and dried plant materials of R. aromatica were also tested separately for essential oil yield and chemistry.
Rhus trilobata Nutt. was collected 16 June 2023, at the Spring Creek Canyon Trail in Springville, Utah 84663, USA (40°09′48.1″ N 111°34′02.8″ W, 1573 m elevation).
Sampling times were chosen to represent early, mid, and late stages of the growing season. Week 1 corresponded to early spring flush, Week 2 to mid-season vegetative growth, Week 3 to late-season leaf maturation, and Week 4 to the beginning of senescence. Not all plant organs were available at every time point. For example, flowers and young shoots were present only during early-season collections, while later collections contained mature leaves and stems without flowers. We have clarified this here because the presence or absence of specific organs may explain part of the compositional differences observed among weekly distillations.

4.2. Extraction and Analysis of Essential Oils

Plant material was added into a steam distillation (Clevenger-type apparatus) to extract the oils and was run for 4–6 h for every distillation. The essential oils were separated from the hydrosol and stored at 4 °C until use. Seasonal yields (Weeks 1–4) are reported on a fresh-weight basis, while drying method yields are expressed on a dry-weight basis. These values are not directly comparable.
Essential oil samples were analyzed on a GC–MS-QP2010 Ultra system (Shimadzu, Columbia, MD, USA). The instrument was operated in electron impact (EI) mode at 70 eV with a scan range of m/z 40–400 and a scan rate of 3.99 scans/s. Separation was achieved on a ZB-5-ms fused-silica capillary column ((5% phenyl)–polydimethylsiloxane stationary phase, 60 m × 0.25 mm i.d., 0.25 μm film thickness). Helium was used as the carrier gas at a constant flow of 1.0 mL/min. The injector temperature was set to 260 °C and the MS source at 260 °C. The oven temperature program was 40 °C, increased at 2 °C/min to 260 °C. Samples were diluted to 5% w/v in dichloromethane, and 0.1 μL was injected using a 30:1 split ratio.
Compounds were identified using a combination of mass spectral matching, retention indices, and authenticated standards. Mass spectra were compared against the SatSet Essential Oil Library [54] as well as the NIST and FFNSC spectral databases. Retention indices (RI) were calculated relative to a homologous series of n-alkanes (C8–C30) under identical chromatographic conditions. Identifications were confirmed by comparison with published literature values and available authenticated reference standards.
Quantitative analyses were performed using a GC-2010 Plus (Shimadzu), Columbus MD, USA, equipped with a flame ionization detector and the same ZB-5-ms column and oven program described above.
Injector and detector temperatures were 260 °C and 280 °C, respectively. Peak areas were integrated using Shimadzu LabSolutions software ver 5.11 conducted as described previously in [55]. Compounds were identified by using retention indices and GC-MS information.
Enantiomeric distributions were determined using a chiral GC column (e.g., Rt-BetaDEXsm, 30 m × 0.25 mm × 0.25 μm). The oven was programmed from 40 °C (5 min) to 200 °C at 2 °C/min. Helium was used as the carrier gas (1.0 mL/min), with injector and detector temperatures at 230 °C and 250 °C. Enantiomers were identified by comparing retention times to authenticated chiral standards purchased from Sigma Aldrich, Saint Louis, MO, USA.
For qualitative volatile analysis, a PDMS SPME fiber (100 μm) was exposed to the headspace of crushed leaf material in water for 30 min at room temperature. Desorption occurred in splitless mode at 250 °C for 1 min. This method was used only for exploratory profiling and interspecific comparison.

4.3. Rhus aromatica and Rhus trilobata Hydrosol Extraction Method

The hydrosol was collected from each steam distillation, and GC-MS analysis was used to identify the compounds present in the hydrosol after extraction. Extraction of the compounds from the hydrosol was performed using dichloromethane (DCM) purchased from Sigma Aldrich, Saint Louis, MO, USA. The hydrosol (100 mL) was extracted with DCM (10 mL) three times using a separatory funnel. DCM was then evaporated to yield the volatile R. aromatica residue, which was subsequently analyzed by GC-MS using the same method as described in Section 2.2.

4.4. Drying Methods of Plant Material

The plant material was collected using the same method as described in Section 2.1. The plant material was dried using three methods: shade-drying, oven-drying, and sun-drying. The shade-dry method involved storing plant material in a dark drawer at ambient temperature for 2 days, allowing all the leaves to dry. The oven-dry method was performed using a drying oven at 48 °C for 6 h, allowing the plant material to dry completely. The sun-drying method was performed by placing the plant material in the sun in mid-July 2023 and drying it for 2 days until completely dry. Each technique was then collected and distilled using the steam distillation method, as described in Section 2.2.

4.5. Solid-Phase Microextraction (SPME) Headspace Method

Three to six leaves of both R. aromatica and R. trilobata samples were added into an amber vial with distilled water, crushed to allow the volatile compounds to be released, and run through Solid-Phase Microextraction (SPME) headspace using a Shimadzu GC-2030 Gas Chromatograph machine, Shimadzu, Columbus, MD, USA coupled with a GCMS-TQ8050 NX Mass Spectrometer. For this specific experiment, the R. trilobata leaves were collected on 20 May 2023, in Battle Creek Canyon, Pleasant Grove, Utah 84062, USA (40°22′09.0″ N, 111°41′16.7″ W, 1828 m elevation).

Author Contributions

Conceptualization, P.S.; methodology, T.B.B.; software, J.E.H.; validation, A.P.; formal analysis, P.S.; investigation, T.B.B. and M.C.R.; resources, P.S.; data curation, T.B.B.; writing—original draft preparation, T.B.B.; writing—review and editing, W.N.S.; supervision, P.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Data Availability Statement

All data are available in the manuscript.

Acknowledgments

The authors would also like to thank Sawyer Ashcroft, Noura Ahmed, and Sushant Sharma Banjara for their additional help with the project. Further thanks to Carlie Beck as well for her support and help during the project.

Conflicts of Interest

Authors Tanner B. Beck, Ambika Poudel, Joseph E. Hilton were employed by the company doTERRA International, LLC. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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