JARS v41n2 - Essential Oils And Rhododendron Scales

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 oils ¹ (Figure 1). In fact, it is the scale-borne essential oils that impart the odor to leaves of certain lepidote species.

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.

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 sesquiterpenes ² , compounds found in rhododendron essential oils.

Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986
Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986 Species Clone Subsection	undefined Total peaks	undefined cam- phene	undefined α- pinene	undefined ß- pinene	undefined ß- myr- cene	undefined 1,8 cin- eole	undefined lina- lool	undefined α- terpi- neal	undefined gera- niol	undefined caryo-phyl-lene	undefined α- humu- lene	undefined cis nero- lidol	undefined trans nero- lidol	undefined β- eleme-none	undefined γ-eudes-mol	undefined α/ß- eudes-mol	undefined ger-ma-crone	undefined farnesol isomers
Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986 edgeworthii Hooker 65.383 Edgeworthia	undefined 18	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined 02	undefined 01	undefined	undefined 10	undefined 01	undefined 03	undefined 26 1	undefined 01
Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986 ciliatum Hooker 65.352 Maddenia	undefined 12	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined 28	undefined 05	undefined	undefined	undefined	undefined	undefined	undefined 03	undefined 45 1
Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986 moupinense Franchet 74.83 Moupinensia	undefined 09	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined 15	undefined	undefined	undefined	undefined	undefined	undefined 10	undefined	undefined
Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986 hanceanum Hemsley 76.34 Tephropepla	undefined 03	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined 39	undefined	undefined	undefined	undefined	undefined	undefined	undefined
Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986 tatsienense Franchet 70.422 Triflora	undefined 06	undefined	undefined 68 1	undefined 20	undefined	undefined	undefined 06	undefined 04	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined
Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986 rigidum Franchet 73.353 Triflora	undefined 07	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined
Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986 keiskei Miquel 76.40 Triflora	undefined 13	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined 30	undefined 09	undefined 06	undefined	undefined 04	undefined 07	undefined	undefined 01
Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986 concinnum Hemsley 73.70 Triflora	undefined 17	undefined 06	undefined	undefined	undefined 27 1	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined 02	undefined 01	undefined 03
Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986 lutescens Franchet 70.107 Triflora	undefined 12	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined 04	undefined 06	undefined	undefined	undefined 09	undefined	undefined 03	undefined 05	undefined 02
Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986 bauhiniiflorum Hutch. 2 73.26 Triflora	undefined 10	undefined 20 1	undefined 05	undefined	undefined	undefined	undefined	undefined	undefined	undefined 13	undefined 06	undefined	undefined 18	undefined	undefined	undefined	undefined	undefined
Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986 trichanthum Rehder 73.280 Triflora	undefined 07	undefined	undefined 06	undefined 15	undefined	undefined 62 1	undefined	undefined	undefined	undefined	undefined 01	undefined	undefined	undefined	undefined	undefined	undefined	undefined
Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986 augustinii Hemsley 77.207 Triflora	undefined 18	undefined	undefined 16 1	undefined	undefined	undefined	undefined	undefined	undefined	undefined 03	undefined	undefined	undefined	undefined 08	undefined	undefined	undefined	undefined
Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986 triflorum Hooker 70.26 Triflora	undefined 13	undefined	undefined	undefined 08	undefined	undefined	undefined	undefined	undefined	undefined 18	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined
Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986 zaleucum Balf. f. & W.W. Sm. 65.405 Triflora	undefined 13	undefined 13	undefined 40 1	undefined	undefined	undefined	undefined	undefined	undefined	undefined 24	undefined 01	undefined	undefined 01	undefined	undefined	undefined	undefined	undefined 01
Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986 davidsonianum Rehd. & Wilson 66.600 Triflora	undefined 08	undefined	undefined 35 1	undefined 23	undefined	undefined	undefined	undefined	undefined	undefined 08	undefined 06	undefined	undefined	undefined	undefined	undefined	undefined	undefined
Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986 yunnanense Franchet 70.333 Triflora	undefined 11	undefined	undefined	undefined	undefined 34 1	undefined	undefined	undefined	undefined	undefined 02	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined
Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986 scabrifolium Franchet 70.155 Scabrifolia	undefined 03	undefined	undefined 90 1	undefined 01	undefined 10	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined
Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986 rubiginosum Franchet 73.130 Heliolepida	undefined 15	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined 14	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined
Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986 carolinianum Rehder 75.133 Caroliniana	undefined 09	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined 10	undefined 30	undefined 01	undefined 07	undefined 46 1	undefined 01
Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986 dauricum L. 66.590	undefined 11	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined 02	undefined	undefined	undefined 01	undefined 09	undefined 06	undefined 11	undefined 49 1	undefined
Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986 chryseum Balf. f. & Ward 3 75.28 Lapponica	undefined 17	undefined	undefined 19	undefined	undefined	undefined	undefined	undefined	undefined	undefined 09	undefined	undefined 14	undefined	undefined	undefined 02	undefined 03	undefined	undefined
Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986 hippophaeoides Balf. f. & Ward 73.135 Lapponica	undefined 20	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined 09	undefined 04	undefined	undefined 18	undefined 17	undefined 18	undefined 19	undefined 39 1	undefined 07
Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986 paludosum Hutch. 4 65.457 Lapponica	undefined 09	undefined	undefined 03	undefined 07	undefined	undefined	undefined	undefined	undefined	undefined 01	undefined 07	undefined	undefined	undefined	undefined	undefined	undefined	undefined
Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986 cuneatum Sm. 65.497 Lapponica	undefined 14	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined 30 1	undefined	undefined	undefined	undefined	undefined 17	undefined	undefined
Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986 polycladum Franchet 65.459 Lapponica	undefined 16	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined 02	undefined	undefined 08	undefined 11	undefined 09	undefined 01	undefined
Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986 nivale Hooker 76.300 Lapponica	undefined 16	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined 03	undefined 05	undefined	undefined 05	undefined 08	undefined 07	undefined	undefined 13	undefined 03
Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986 russatum Balf. f. & Forr. 73.24 Lapponica	undefined 17	undefined	undefined 20 1	undefined	undefined 06	undefined	undefined	undefined	undefined 15	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined
Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986 impeditum Balf. f. & W.W. Sm. 76.102 Lapponica	undefined 18	undefined	undefined 11	undefined 03	undefined 04	undefined	undefined	undefined 01	undefined	undefined 08	undefined 07	undefined	undefined 17 1	undefined 14	undefined 05	undefined 06	undefined 11	undefined 02
Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986 dasypetalum Balf. f. & Forr. 74.70 Lapponica	undefined 12	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined 06	undefined 04	undefined	undefined 02	undefined 14	undefined 02	undefined
Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986 intricatum Franchet 73.144 Lapponica	undefined 08	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined 06	undefined	undefined 03	undefined	undefined	undefined	undefined 25	undefined	undefined
Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986 capitatum Maxim. 74.64 Lapponica	undefined 09	undefined	undefined 32	undefined 47 1	undefined	undefined	undefined	undefined	undefined	undefined	undefined 02	undefined	undefined	undefined	undefined	undefined	undefined	undefined
Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986 ferrugineum L 76.381 Rhododendron	undefined 18	undefined	undefined 20	undefined 15	undefined	undefined	undefined	undefined	undefined 01	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined
Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986 micranthum Turcz 76.399 Micrantha	undefined 09	undefined	undefined	undefined 16	undefined	undefined	undefined	undefined	undefined	undefined 03	undefined	undefined	undefined	undefined	undefined	undefined 09	undefined	undefined
Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986 calostrotum Balf. f. & Ward 66.573 Saluenensia	undefined 20	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined 02	undefined 11	undefined 07	undefined 08	undefined 04	undefined 07	undefined 09	undefined 08	undefined 05
Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986 pemakocnse Ward 70.42 Uniflora	undefined 10	undefined	undefined 06	undefined 09	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined
Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986 xanthocodon Hutch. 5 73.305 Cinnabarina	undefined 18	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined 10	undefined 07	undefined 03	undefined	undefined
Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986 virgatum Hooker 65.404 Virgata	undefined 11	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined 38	undefined 02	undefined 03	undefined 09	undefined	undefined 08	undefined 10	undefined	undefined 12
Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986 glaucophyllum Rehder 76.98 Glauca	undefined 16	undefined	undefined	undefined	undefined 20	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined 04	undefined 07	undefined	undefined
Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986 glaucophyllum var. luteiflorum Rehder 6 64.114 Glauca	undefined 12	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined 03	undefined 09	undefined 06	undefined 04	undefined 05	undefined 02	undefined 05	undefined 28 1	undefined
Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986 campylogynum Franchet 74.62 Camphylogyna	undefined 12	undefined	undefined	undefined	undefined 08	undefined	undefined	undefined	undefined	undefined	undefined 05	undefined 03	undefined 60 1	undefined	undefined 02	undefined	undefined	undefined
Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986 lepidotum Wallich 79.53 Lepidota	undefined 13	undefined	undefined 50	undefined 22	undefined	undefined	undefined	undefined	undefined	undefined 01	undefined	undefined	undefined	undefined	undefined	undefined 03	undefined	undefined
Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986 baileyi Balf 64.146 Baileya	undefined 13	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined 12	undefined 09	undefined	undefined	undefined	undefined 02	undefined	undefined	undefined
Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986 leucaspis Tagg 65.398 Boothia	undefined 03	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined	undefined
Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986 rubrolineatum Balf. f. & Forr. 7 76.205 Trichoclada	undefined 08	undefined	undefined 19	undefined 14	undefined	undefined	undefined 18	undefined	undefined	undefined	undefined 04	undefined	undefined	undefined	undefined	undefined	undefined	undefined
Table 1.  Components Present In Essential Oils from 43 Lepidote Rhododendron Species Taken with permission from Phytochemistry, 1986 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.