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Journal American Rhododendron Society

Current Editor:
Dr. Glen Jamieson ars.editor@gmail.com

Essential Oils And Rhododendron Scales
Robert P. Doss, Ph.D.
U.S. Department of Agriculture
Agricultural Research Service and Department of Horticulture
Oregon State University, Corvallis, Oregon

        Rhododendron fanciers are familiar with the scales borne on the leaves of the lepidote rhododendrons. Scale morphology serves as an important character in classifying the lepidote species. What may be less well-known is that the scales are rich sources of essential oils¹ (Figure 1). In fact, it is the scale-borne essential oils that impart the odor to leaves of certain lepidote species.

Figure 1. A detached scale (about 0.3 mm in diameter) from R. chryseum showing oil droplets (arrows). Taken, with permission, from J. Chem. Ecol. 10:1787 (1984).

        The volatile compounds in rhododendron scales are responsible for the resistance to adult root weevil feeding exhibited by some lepidote plants (Figure 2). The most resistant species are those with leaves containing the largest amounts of such materials.

Figure 2. Obscure root weevils are repelled by germacrone, a material in the essential oil from R. edgeworthii, that has been applied (0.05 mg) to the filter paper disk on the right.

        The amounts of essential oils present in the leaves are determined by the density of scales and the amount of oil present in each scale. Scale density, in one group of 11 species varied from 109 scales per square centimeter on R. rigidum, which has scales only on the lower surface, to 4189 scales per square centimeter on R. chryseum, which has scales on both surfaces. Essential oil contents in this same group varied from 25 mg/scale with R. lepidotum to 151 mg/scale with R. edgeworthii.
        Just as scales from different species vary in morphology, they also vary with respect to the chemical compounds making up the essential oils (Table 1). The oils are often very complex, some containing 20 compounds at fairly high concentration, and many more compounds at lower concentration. Many of the materials detected have not been identified. The materials listed in Table 1 are monoterpenes and sesquiterpenes², compounds found in rhododendron essential oils.

Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986
Species Clone Subsection	Total peaks	cam- phene	α- pinene	ß- pinene	ß- myr- cene	1,8 cin- eole	lina- lool	α- terpi- neal	gera- niol	caryo-phyl-lene	α- humu- lene	cis nero- lidol	trans nero- lidol	β- eleme-none	γ-eudes-mol	α/ß- eudes-mol	ger-ma-crone	farnesol isomers
edgeworthii Hooker 65.383 Edgeworthia	18										02	01		10	01	03	261	01
ciliatum Hooker 65.352 Maddenia	12									28	05						03	451
moupinense Franchet 74.83 Moupinensia	09									15						10
hanceanum Hemsley 76.34 Tephropepla	03										39
tatsienense Franchet 70.422 Triflora	06		681	20			06	04
rigidum Franchet 73.353 Triflora	07
keiskei Miquel 76.40 Triflora	13										30	09	06		04	07		01
concinnum Hemsley 73.70 Triflora	17	06			271											02	01	03
lutescens Franchet 70.107 Triflora	12									04	06			09		03	05	02
bauhiniiflorum Hutch.2 73.26 Triflora	10	201	05							13	06		18
trichanthum Rehder 73.280 Triflora	07		06	15		621					01
augustinii Hemsley 77.207 Triflora	18		161							03				08
triflorum Hooker 70.26 Triflora	13			08						18
zaleucum Balf. f. & W.W. Sm. 65.405 Triflora	13	13	401							24	01		01					01
davidsonianum Rehd. & Wilson 66.600 Triflora	08		351	23						08	06
yunnanense Franchet 70.333 Triflora	11				341					02
scabrifolium Franchet 70.155 Scabrifolia	03		901	01	10
rubiginosum Franchet73.130 Heliolepida	15									14
carolinianum Rehder 75.133 Caroliniana	09												10	30	01	07	461	01
dauricum L. 66.590	11									02			01	09	06	11	491
chryseum Balf. f. & Ward3 75.28 Lapponica	17		19							09		14			02	03
hippophaeoides Balf. f. & Ward 73.135 Lapponica	20									09	04		18	17	18	19	391	07
paludosum Hutch.465.457 Lapponica	09		03	07						01	07
cuneatum Sm. 65.497 Lapponica	14										301					17
polycladum Franchet 65.459 Lapponica	16											02		08	11	09	01
nivale Hooker 76.300 Lapponica	16									03	05		05	08	07		13	03
russatum Balf. f. & Forr. 73.24 Lapponica	17		201		06				15
impeditum Balf. f. & W.W. Sm. 76.102 Lapponica	18		11	03	04			01		08	07		171	14	05	06	11	02
dasypetalum Balf. f. & Forr. 74.70 Lapponica	12											06	04		02	14	02
intricatum Franchet 73.144 Lapponica	08									06		03				25
capitatum Maxim. 74.64 Lapponica	09		32	471							02
ferrugineum L 76.381 Rhododendron	18		20	15					01
micranthum Turcz 76.399 Micrantha	09			16						03						09
calostrotum Balf. f. & Ward 66.573 Saluenensia	20									02	11	07	08	04	07	09	08	05
pemakocnse Ward 70.42 Uniflora	10		06	09
xanthocodon Hutch.5 73.305 Cinnabarina	18													10	07	03
virgatum Hooker 65.404 Virgata	11									38	02	03	09		08	10		12
glaucophyllum Rehder 76.98 Glauca	16				20										04	07
glaucophyllum var. luteiflorum Rehder6 64.114 Glauca	12									03	09	06	04	05	02	05	281
campylogynum Franchet 74.62 Camphylogyna	12				08						05	03	601		02
lepidotum Wallich 79.53 Lepidota	13		50	22						01						03
baileyi Balf 64.146 Baileya	13									12	09				02
leucaspis Tagg 65.398 Boothia	03
rubrolineatum Balf. f. & Forr.7 76.205 Trichoclada	08		19	14			18				04
1 Largest peak in extract. 2 R. bauhiniiflorum is considered a variety of R. triflorum Hooker by some authors. 3 R. chryseum is considered a variety of R. rupicola W.W. Sm. by some authors. 4 R paludosum is considered to be the same as R. nivale Hooker, subspecies nivale by some authors. 5 R. xanthocodon is considered a subspecies of R. cinnabarinum by some authors. 6 R. glaucophyllum var. luteiflorum is considered to be R. luteiflorum Cullen by some authors. 7 R. rubrolineatum is considered a variety of R. mekongense Franchet by some authors.

Clone numbers referred to in Table 1 are Rhododendron Species Foundation numbers. The peak numbers shown on the Table are estimates of the percentage composition of the particular oil present in the sample.
        The materials shown in Table 1, with a few exceptions, are common constituents of essential oils of other plants. For example, humulene and caryophyllene are found in hops, the eudesmols are found in eucalyptus, and the pinenes are found in leaves of many plant species. The complexity and variation of the essential oils of different rhododendron species suggest that, like scale morphology, oil composition could be used in classification.
        Before the usefulness of essential oil profiles can be tested, it will be necessary to identify more of the compounds making up the oils, including materials found at only trace levels. With such information it may be possible to improve the taxonomic treatment of the lepidote rhododendrons.

        Acknowledgements: Some of the information in this report was obtained with support from the American Rhododendron Society Research Foundation. Contribution of the Agricultural Research Service, U.S. Department of Agriculture, in cooperation with the Agriculture Experiment Station, Oregon State University. Technical Paper No. 7938 of the latter.

¹ Essential oils are volatile oils found in plants that often possess a strong odor. These volatile oils are frequently a mixture of many compounds.
² Monoterpenes and sesquiterpenes are compounds containing 10 and 15 carbon atoms, respectively, and are formed from 5 carbon atom units (isoprene units) via the terpenoid biosynthetic pathway, an important biosynthetic pathway in plants.

References:
Doss, R.P., R. Luthi, and B.F. Hrutfiord. 1980. Cermacrone, a sesquiterpene repellent to obscure root weevil from Rhododendron edgeworthii. Phytochemistry 19:2379-2380.
Doss, R.P. 1984. Role of glandular scales of lepidote rhododendrons in insect resistance. J. Chem. Ecol. 10:1787-1798.
Doss, R.P., W.H. Hatheway, and B.F. Hrutfiord. 1986. Composition of essential oils of some lepidote Rhododendrons. Phytochemistry 25:1637-1640.

Dr. Doss is plant physiologist at the Horticultural Crops Research Laboratory, Corvallis, Oregon, specializing in the physiology of flowering and plant defenses.