1.    Introduction

Numerous members of the Asteraceae growing in desert regions of western North America have been used as traditional herbal medicines by Native American tribes [1]. Two members of the family, Chaenactis douglasii (Hook.) Hook. & Arn (Douglas’ dustymaiden) and Dieteria canescens (Pursch) Nutt. (syn. Machaeranthera canescens (Pursch) A. Gray, hoary tansyaster) have been understudied in terms of their volatile phytochemicals.

Chaenactis douglasii occurs in western North America from British Columbia, south to southern California, northern Arizona and northern New Mexico, and east into Montana, Wyoming, and Colorado [2, 3]. Several Native North American tribes used C. douglasii as traditional medicines. For example, the Paiute took a decoction of the plant to treat coughs and colds, the Shoshoni used a poultice of crushed plants to treat swellings, and the Thompson people used a decoction of the plant on skin conditions and insect bites [1]. Guaianolide and germacranolide sesquiterpene lactones have been isolated and identified in the aerial parts of C. douglasii [4–6]. The crude methanol extract (not phytochemically characterized) of C. douglasii from British Columbia, Canada, was screened for antibacterial [7] and antifungal [8] activity. The extract showed activity against Bacillus subtilis, Escherichia coli, Mycobacter phlei, both methicillin-susceptible and methicillin-resistant Staphylococcus aureus, Salmonella typhimurium, Microsporum cookerii, Microsporum gypseum, and Trichophyton mentagrophytes. Apparently there have been no previous reports on the essential oil composition of C. douglasii, however.

Dieteria canescens ranges throughout western North America from Canada, south through California, Arizona, and New Mexico, to Mexico [3, 9]. The Navajo used the dried, pulverized plant material as a snuff to treat throat problems, while a decoction of the plant was taken by parturient Hopi women for any disorder [1].  As far as we are aware, there are no previous studies on the phytochemistry of D. canescens.

Both C. douglasii and D. canescens are important native members of the plant communities of the Great Basin [10]. However, these native plant communities of the Great Basin are being lost or degraded at an alarming rate due to overgrazing, wildfires, drought, and invasive species; the sagebrush steppe is one of the most endangered ecosystems of North America [11,12]. Although relatively understudied, C. douglasii [13] and D. canescens [14] have been highlighted as priority species for restoration of degraded habitats in the Great Basin. However, neither plant has been investigated in terms of essential oil composition. As part of our continuing interest in essential oils of aromatic/medicinal plants of the Great Basin, the purpose of this study was to investigate the essential oil compositions of these two members of the Asteraceae, C. douglasii and D. canescens collected from Boise, Idaho. 

2.    Materials and methods

2.1 Plant material

Aerial parts of C. douglasii and D. canescens were collected from the Idaho Botanical Garden (43°36ʹ04″N, 116°09ʹ35″W, 862 m elevation) on 29 July 2021. The plants were identified by Daniel Murphy, Collections Curator, Idaho Botanical Garden. The fresh aerial parts of C. douglasii (17.34 g) were hydrodistilled using a Likens-Nickerson apparatus with continuous extraction with dichloromethane for 3 h to give a colorless residue (4.1 mg). The fresh aerial parts of D. canescens (48.19 g) were hydrodistilled for 3 h to give a pale-yellow essential oil (201.2 mg).

2.2 Gas chromatographic analyses

The aerial parts essential oils of C. douglasii and D. canescens were analyzed by GC-MS as previously reported [15]: Shimadzu GCMS-QP2010 Ultra (Shimadzu Scientific Instruments, Columbia, MD, USA), ZB-5ms capillary column (60 m × 0.25 mm, 0.25 μm film thickness, Phenomenex, Torrance, CA, USA); He carrier gas, head pressure = 208.3 kPa, flow rate = 2.00 mL/min, injector temperature = 260 °C, ion source temperature = 260 °C, interface temperature = 260 °C, GC oven program (50 °C initial temperature, ramp up to 260 °C at 2 °C/min, held at 260 °C for 5 min); 0.1 μL injection, 5% w/v essential oil/CH₂Cl₂, 24.5:1 split mode. A series of homologous n-alkanes was used to calculate the retention indices (RI) [16]. Chemical components were identified by comparing MS fragmentation and RI values with those in the Adams [17], FFNSC3 [18], NIST20 [19], and Satyal [20] databases. Gas chromatography – flame ionization detection (GC-FID) was carried out as previously described [15]: Shimadzu GC 2010 with FID detector (Shimadzu Scientific Instruments, Columbia, MD, USA), ZB-5 capillary column (60 m ´ 0.25 mm ´ 0.25 μm film thickness) (Phenomenex, Torrance, CA, USA), same operating conditions as above for GC-MS. The percent compositions were determined from raw peak areas without standardization. Chiral GC-MS was used to evaluate the enantiomeric distributions of chiral terpenoids as previously reported [15]: Shimadzu GCMS-QP2010S (Shimadzu Scientific Instruments, Columbia, MD, USA), Restek B-Dex 325 column (30 m ´ 0.25 mm diameter ´ 0.25 μm film thickness) (Restek Corp., Bellefonte, PA, USA); He carrier gas, head pressure = 53.6 kPa, flow rate = 1.00 mL/min, injector and detector temperatures = 240 °C, GC oven program (50 °C initial temperature held for 5 min, ramp up to 100 °C at 1.0 °C/min, then ramp to 220 °C at 2.0 °C/min); 0.3 μL injection, 5% w/v essential oil/CH₂Cl₂, 24.0:1 split mode. Retention indices (RI) determined with respect to a series (C₈-C₂₁) of n-alkanes. Enantiomers were determined by comparison of RI values with those in our in-house database. The enantiomer percentages were determined from raw peak areas.

 

3.    Results and discussion

Hydrodistillation of the aerial parts of C. douglasii gave a colorless essential oil in very low (0.024%) yield while the aerial parts of D. canescens gave a pale-yellow essential oil in 0.418% yield.  Gas chromatographic analyses (GC-MS and GC-FID) of the two essential oils are summarized in Table 1.

 

Table 1. Chemical composition (%) of the aerial parts essential oil of Chaenactis douglasii and Dieteria canescens. 

RIcalc	RIdb	Compound	C. douglasii	D. canescens
933	933	α-Pinene	0.2	0.4
941	942	Tiglic acid	35.9	-
949	950	Camphene	-	0.1
971	971	Sabinene	-	tr
978	978	β-Pinene	1.6	1.3
979	979	Caproic acid	0.7	-
988	989	Myrcene	-	0.3
1007	1007	α-Phellandrene	-	tr
1017	1018	α-Terpinene	-	tr
1024	1024	p-Cymene	-	0.5
1026	1026	2-Acetyl-3-methylfuran	-	tr
1029	1030	Limonene	-	0.5
1030	1031	β-Phellandrene	-	tr
1031	1032	1,8-Cineole	-	tr
1035	1035	(Z)-β-Ocimene	-	tr
1045	1045	(E)-β-Ocimene	-	tr
1057	1057	γ-Terpinene	-	tr
1069	1069	cis-Sabinene hydrate	-	0.2
1085	1086	Terpinolene	-	tr
1100	1101	trans-Sabinene hydrate	-	0.1
1107	1106	α-Pinene oxide	-	tr
1121	1121	trans-p-Mentha-2,8-dien-1-ol	-	tr
1124	1126	cis-p-Menth-2-en-1-ol	-	0.1
1137	1137	Nopinone	-	tr
1141	1142	trans-p-Menth-2-en-1-ol	-	0.1
1145	1145	trans-Verbenol	-	0.2
1149	1146	trans-Limonene oxide	-	tr
1161	1164	Pinocarvone	-	tr
1171	1170	Borneol	-	tr
1175	1176	cis-Pinocamphone	-	tr
1180	1180	Terpinen-4-ol	-	1.2
1186	1186	p-Cymen-8-ol	-	0.1
1195	1195	α-Terpineol	0.3	0.1
1196	1196	cis-Piperitol	-	tr
1206	1206	Verbenone	-	tr
1208	1208	trans-Piperitol	-	tr
1283	1282	Bornyl acetate	0.5	0.1
1289	1289	Thymol	8.4	-
1348	1348	α-Cubebene	-	0.7
1376	1375	α-Copaene	-	0.4
1384	1382	β-Bourbonene	-	0.1
1387	1385	α-Isocomene	0.2	-
1390	1392	β-Cubebene	-	0.9
1407	1406	α-Gurjunene	-	0.1
1418	1422	β-Ylangene	-	tr
1419	1417	(E)-β-Caryophyllene	-	0.1
1446	1447	Geranyl acetone	0.3	-
1449	1448	cis-Muurola-3,5-diene	-	0.1
1454	1454	α-Humulene	0.2	tr
1465	1464	Ylangol	-	tr
1469	1463	cis-Cadina-1(6),4-diene	-	tr
1472	1472	trans-Cadina-1(6),4-diene	-	0.6
1475	1475	γ-Muurolene	-	0.1
1476	1481	(E)-β-Ionone	0.1	-
1482	1483	Germacrene D	0.7	0.2
1489	1489	β-Selinene	-	0.1
1492	1492	trans-Muurola-4(14),5-diene	0.7	1.9
1496	1497	epi-Cubebol	2.3	24.4
1497	1497	α-Muurolene	0.3	-
1511	1510	Tridecanal	0.5	-
1516	1515	Cubebol	0.9	2.9
1518	1518	δ-Cadinene	5.0	28.3
1520	1519	trans-Calamenene	0.6	-
1522	1521	Zonarene	0.3	-
1523	1524	Dihydroactinidiolide	0.3	-
1526	1527	trans-Calamenene	-	3.8
1533	1533	trans-Cadine-1,4-diene	0.4	0.6
1542	1541	α-Calacorene	-	0.4
1549	1549	α-Elemol	2.9	0.3
1560	1561	(E)-Nerolidol	0.3	-
1562	1564	β-Calacorene	-	0.3
1570	1568	Palustrol	-	0.2
1575	1576	Spathulenol	0.4	-
1584	1590	Globulol	0.3	-
1588	1590	Gleenol	-	1.4
1590	-	Unidentifieda	-	1.8
1592	-	Unidentifiedb	-	2.7
1593	1594	Viridiflorol	0.2	-
1600	1600	Hexadecane	0.2	-
1613	1613	Tetradecanal	0.2	-
1622	1624	Selina-6-en-4β-ol	0.9	-
1629	1628	1-epi-Cubenol	3.2	10.0
1631	1632	γ-Eudesmol	0.9	0.1
1643	1643	Cubenol	2.3	7.7
1643	1644	τ-Muurolol	0.9	-
1646	1644	α-Muurolol (= δ-Cadinol)	0.7	1.9
1654	1655	α-Cadinol	3.8	-
1657	1663	cis-Calamenen-10-ol	-	0.5
1663	1665	Intermedeol	1.8	-
1664	1662	9-Methoxycalamenene	-	0.4
1714	1715	Pentadecanal	0.4	-
1757	1758	Myristic acid	0.3	-
1777	1775	8α-Acetoxyelemol	4.7	0.2
1792	1793	α-Phellandrene dimer A	0.5	-
1840	1841	Phytone	0.2	-
1889	1891	(E)-Hexadecantrienal	0.7	-
1957	1958	Palmitic acid	0.3	-
1998	-	Unidentifiedc	8.7	-
		Total identified	86.4	93.9
Compound Classes
Monoterpene hydrocarbons			1.8	3.0
Oxygenated monoterpenoids			9.1	2.2
Sesquiterpene hydrocarbons			8.4	38.7
Oxygenated sesquiterpenoids			26.7	49.9
Diterpenoids			0.5	0.0
Others			39.9	traces

RI_calc = Retention index calculated with respect to a homologous series of n-alkanes on a ZB-5ms column. RI_db = Reference retention index values from the databases [17–20]. tr = trace (< 0.05%), - = not detected. ^a MS(EI): 222(3%), 208(13%), 207(90%), 204(17%), 161(57%), 137(18%), 135(21%), 123(25%), 119(22%), 105(38%), 95(26%), 91(18%), 55(17%), 43(100%), 41(24%). ^b MS(EI): 222(2%), 208(14%), 207(100%), 179(9%), 161(37%), 135(19%), 123(23%), 119(22%), 105(35%), 95(26%), 91(21%), 81(27%), 55(18%), 43(88%), 41(26%). ^c MS(EI): 230(13%), 215(8%), 186(8%), 169(8%), 157(11%), 145(84%), 143(22%), 129(23%), 117(23%), 105(30%), 194(100%), 91(59%), 79(44%), 77(35%), 65(17%), 53(14%), 41(24%).

Tiglic acid (35.9%) dominated the essential oil of C. douglasii, followed by thymol (8.4%), δ-cadinene (5.0%), and 8α-acetoxyelemol (4.7%). There was also a large concentration of an unidentified component (RI = 1998, 8.7%). Although the essential oil yield and the terpenoid content were low, the enantiomeric distributions for the monoterpenoids α-pinene, β-pinene, and α-terpineol, as well as the sesquiterpenoids germacrene D, δ-cadinol, and (E)-nerolidol, were determined (Table 2). (+)-α-Pinene was the predominant enantiomer (enantiomeric excess, %ee, = 51.4%) and α-terpineol was nearly racemic [52.4% (+), 47.6% (–)]; the (–) enantiomers were found exclusively for β-pinene and germacrene D, while the (+)-enantiomers were observed for δ-cadinene, and (E)-nerolidol.

The major components in D. canescens essential oil were δ-cadinene (28.3%), epi-cubebol (24.4%), 1-epi-cubenol (10.9%), and cubenol (7.7%). The levorotatory enantiomers were dominant for the monoterpenes α-pinene (ee 80.6%), β-pinene (ee 80.8%), camphene (ee 84.2%), cis-sabinene hydrate (ee 73.8%) trans-sabinene hydrate (ee 70.0%), terpinen-4-ol (ee 33.6%), and α-terpineol (ee 29.6%). On the other hand, (+)-limonene was the major limonene enantiomer (ee 91.6%). As observed in C. douglasii, the (–) enantiomer was the exclusive stereoisomer for the germacrene D, while (+)-δ-cadinene was the only enantiomer observed in D. canescens.

 

Table 2. Enantiomeric distributions of chiral terpenoids detected in Chaenactis douglasii

and Dieteria canescens essential oils.

Compound	Database	C. douglasii EO		D. canescens EO
	RT (RI)	RT (RI)	ED (%)	RT (RI)	ED (%)
(–)-α-Pinene	15.92 (976)	16.34 (976)	24.3	16.36 (976)	90.3
(+)-α-Pinene	16.40 (982)	16.78 (981)	75.7	16.86 (983)	9.7
(+)-β-Pinene	20.27 (1027)	-	0.0	20.80 (1027)	9.6
(–)-β-Pinene	20.62 (1030)	21.36 (1032)	100.0	21.22 (1031)	90.4
(–)-Limonene	25.06 (1073)	-	-	25.73 (1074)	4.2
(+)-Limonene	25.99 (1081)	-	-	26.47 (1080)	95.8
(+)-cis-Sabinene hydrate	40.70 (1199)	-	-	41.48 (1198)	13.1
(–)-cis-Sabinene hydrate	41.25 (1202)	-	-	41.98 (1200)	86.9
(+)-trans-Sabinene hydrate	46.15 (1230)	-	-	46.96 (1231)	15.0
(–)-trans-Sabinene hydrate	46.84 (1235)	-	-	47.63 (1235)	85.0
(+)-Terpinen-4-ol	54.64 (1297)	-	-	55.40 (1299)	33.2
(–)-Terpinen-4-ol	54.93 (1299)	-	-	55.70 (1302)	66.8
(–)-α-Terpineol	59.73 (1347)	60.34 (1348)	47.6	60.45 (1349)	64.8
(+)-α-Terpineol	60.58 (1356)	61.14 (1356)	52.4	61.24 (1358)	35.2
(+)-Germacrene D	73.48 (1518)	-	0.0	-	0.0
(–)-Germacrene D	73.73 (1522)	74.01 (1521)	100.0	74.10 (1522)	100.0
(–)-δ-Cadinene	76.50 (1563)	-	0.0	-	0.0
(+)-δ-Cadinene	77.33 (1576)	77.67 (1576)	100.0	77.53 (1574)	100.0
(–)-(E)-Nerolidol	83.40 (1677)	-	0.0	-	-
(+)-(E)-Nerolidol	83.59 (1680)	83.96 (1681)	100.0	-	-

RT = Retention time (min). RI = Retention index calculated with respect to a homologous series of n-alkanes on a

Restek B-Dex 325 column. EO = Essential oil. ED = Enantiomeric distribution. - = not observed.

There have been no previous reports on the chemical compositions of either C. douglasii or D. canescens essential oils, so direct comparisons cannot be made. Nevertheless, comparisons with other members of the Asteraceae are possible. 

Although tiglate esters occur in many essential oils, the occurrence of the free carboxylic acid is rare apparently [21]. Nevertheless, tiglic acid was found in relatively large concentration (18.9%) in the essential oil of Ajuga orientalis (Lamiaceae) from Jordan [22]. The Lamiaceae is known to be a rich source of thymol [23], especially Thymus vulgaris [24], Monarda spp. [25, 26], Origanum tyttanthum [27], and Satureja intermedia [28]. Although the distribution of thymol in the Asteraceae is limited [29], several members of the family have shown relatively high concentrations of thymol in their essential oils, including, for example, Phagnalon sordidum (1.3-11.0%) [30], Tanacetum sinaicum (17.0-18.7%) [31], Tanacetum walteri (22.5%) [32], and Tridex procumbens (20.9-68.9%) [33]. 

Numerous members of the Asteraceae have been shown to be rich sources of sesquiterpenoids such as (E)-β-caryophyllene (Eclipta prostrata, 47.7% [34]; Duhaldea cappa, 27.5% [35]; and Smallanthus uvedalia, 16.5-24.5% [36]) and germacrene D (Polymnia canadensis, 44.5-63.6% [36]; Verbesina turbacensis, 29.1-36.9% [37]; and Blumea lacera 25.5% [38]). Additionally, comparable to D. canescens essential oil, Calendula arvensis essential oil, depending on the chemotype, has shown high concentrations of epi-cubebol (1.1-15.2%), δ-cadinene (6.0-18.1%), 1-epi-cubenol (0.5-16.3%), and cubenol (0.3-9.2%) [39, 40].

Several essential oils of members of the Asteraceae have been analyzed by chiral GC-MS to evaluate their enantiomeric distributions (Table 3). There do not seem to be any obvious trends in enantiomeric distribution of monoterpene hydrocarbons in the family. (–)-α-Pinene seems to predominate in essential oils of Baccharis and Gnoxys essential oils, but is variable in Artemisia species. The distribution of α-pinene is not consistent within samples of Artemisia annua [41]. In the case of β-pinene, the enantiomeric distribution is inconsistent within the genera Artemisia, Baccharis, and Gnoxys. (+)-Limonene predominated in the essential oils of Baccharis species, but were not consistent in Ericameria nauseosa [42]. The oxygenated monoterpenoids terpenen-4-ol and α-terpineol also do not exhibit consistent enantiomeric distributions in the family. There are too few data available for the sesquiterpenes to draw any conclusive trends.

 Table 3. Enantiomeric distribution of terpenoids in essential oils of the Asteraceae. 

Essential oil	α-Pinene		β-Pinene		Limonene		cis-Sabinene hydrate		trans-Sabinene hydrate		Terpinen-4-ol		α-Terpineol		Germacrene D		δ-Cadinene		(E)-Nerolidol		Ref.
	(+)	(–)	(+)	(–)	(+)	(–)	(+)	(–)	(+)	(–)	(+)	(–)	(+)	(–)	(+)	(–)	(+)	(–)	(+)	(–)
Achillea ligustica	59.0	41.0	2.0	98.0	-	-	-	-	-	-	56.0	44.0	-	-	-	-	-	-	-	-	[43]
Achillea millefolium	3.0-79.0	21.0-97.0	0.0-2.0	98.0-100.0	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-	-	[44]
Artemisia annua	20.0-96.4	3.6-80.0	0.0	100.0	0.0	100.0	-	-	-	-	-	-	-	-	-	-	-	-	-	-	[41]
Artemisia dracunculus	96.2	3.9	34.0	66.0	78.0	22.0	-	-	-	-	-	-	43.4	56.6	0.0	100.0	-	-	0.0	100.0	[45]
Artemisia tridentata	100.0	0.0	100.0	0.0	-	-	-	-	-	-	25.7-29.8	70.2-74.3	-	-	-	-	0.0	100.0	7.9-8.1	91.9-92.1	[46]
Baccharis dracunculifolia	31.0-44.4	55.6-69.0	25.4-43.8	56.2-74.6	63.0-80.5	19.5-37.0	-	-	-	-	35.0-42.0	58.0-65.0	51.0-68.0	32.0-49.0	-	-	-	-	-	-	[47]
Baccharis microdonta	33.1-36.6	63.4-66.9	41.7-49.9	50.1-58.3	70.2-85.4	14.6-29.8	-	-	-	-	30.0-38.9	61.1-70.0	56.0-81.0	19.0-44.0	-	-	-	-	-	-	[47]
Baccharis tridentata	4.0	96.0	73.0	27.0	66.2	33.8	-	-	-	-	-	-	-	-	-	-	-	-	-	-	[48]
Chaenactis douglasii	75.7	24.3	0.0	100.0	-	-	-	-	-		-	-	52.4	47.6	0.0	100.0	100.0	0.0	100.0	0.0	[This work]
Chrysothamnus viscidiflorus	5.5	94.5	0.2	99.8	92.3	7.7	86.1	13.9	90.5	9.5	71.9	28.1	12.6	87.4	33.4	66.6	-	-	-	-	[15]
Coreopsis triloba	18.4	81.6	100.0	0.0	2.5	97.5	-	-	-	-	-	-	-	-	0.0	100.0	-	-	-	-	[49]
Dieteria canescens	9.7	90.3	9.6	90.4	95.8	4.2	13.1	86.9	15.0	85.0	33.2	66.8	35.2	64.8	0.0	100.0	100.0	0.0	-	-	[This work]
Diplosthephium juniperinum	100.0	0.0	3.4	96.6	-	-	-	-	-	-	71.6	28.4	-	-	7.7	92.3	-	-	-	-	[50]
Erechtites hieracifolia	100.0	0.0	10.3	89.7	0.0	100.0	-	-	-	-	-	-	-	-	-	-	-	-	-	-	[51]
Ericameria nauseosa	9.4-41.5	59.5-90.6	0.4-10.4	89.6-99.6	4.2-59.8	40.2-95.8	19.8-22.7	77.3-80.2	20.9-22.1	77.9-79.1	31.4-34.1	65.8-68.6	0.0-32.7	67.3-100.0	-	-	-	-	-	-	[42]
Gynoxys buxifolia	0.0	100.0	0.0	100.0	-	-	-	-	-	-	0.0	100.0	-	-	-	-	-	-	-	-	[52]
Gynoxys miniphylla	0.9	99.1	44.1	55.9	-	-	-	-	-	-	-	-	-	-	4.5	95.5	-	-	-	-	[53]
Gynoxys rugulosa	37.1	62.9	100.0	0.0	-	-	-	-	-	-	57.4	42.6	49.9	50.1	95.5	4.5	-	-	-	-	[54]
Solidago canadensis	73.9	26.1	36.4	63.6	97.7	2.3	-	-	-	-	-	-	-	-	72.4	27.6	-	-	-	-	[55]
Tagetes maxima	88.3	11.7	57.6	42.4	95.6	4.4	-	-	-	-	-	-	-	-	-	-	-	-	-	-	[56]

 

4.    Conclusions

This is the first report on the volatile phytochemicals found in Chaenactis douglasii and Dieteria canescens. The essential oil yield for C. douglasii was very low and the essential oil was dominated by tiglic acid and thymol. The presence of thymol may account for the reported antimicrobial effects of C. douglasii extracts. The essential oil of D. canescens, however, was rich in both sesquiterpene hydrocarbons and oxygenated sesquiterpenoids. The enantiomeric distributions of chiral monoterpenoids in these essential oils do not show trends consistent with other members of the Asteraceae. Several genotypes (ecotypes) have been described for C. douglasii and D. canescens, so it is likely that different chemotypes are also possible. Future research should be carried out on C. douglasii and D. canescens from other geographical locations and habitats to describe the phytochemical types of these species more completely.

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 Daniel Murphy, Collections Curator, Idaho Botanical Garden, for identification of the plants in the Garden. 

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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