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.

R. chryseum scale and oil droplet
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.

Root weevils repelled by germacrone
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 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
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 26 1 01
ciliatum Hooker
65.352
Maddenia
12 28 05 03 45 1
moupinense Franchet
74.83
Moupinensia
09 15 10
hanceanum Hemsley
76.34
Tephropepla
03 39
tatsienense Franchet
70.422
Triflora
06 68 1 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 27 1 02 01 03
lutescens Franchet
70.107
Triflora
12 04 06 09 03 05 02
bauhiniiflorum Hutch. 2
73.26
Triflora
10 20 1 05 13 06 18
trichanthum Rehder
73.280
Triflora
07 06 15 62 1 01
augustinii Hemsley
77.207
Triflora
18 16 1 03 08
triflorum Hooker
70.26
Triflora
13 08 18
zaleucum
Balf. f. & W.W. Sm. 65.405
Triflora
13 13 40 1 24 01 01 01
davidsonianum Rehd. & Wilson
66.600
Triflora
08 35 1 23 08 06
yunnanense Franchet 70.333
Triflora
11 34 1 02
scabrifolium Franchet 70.155
Scabrifolia
03 90 1 01 10
rubiginosum Franchet 73.130
Heliolepida
15 14
carolinianum Rehder
75.133
Caroliniana
09 10 30 01 07 46 1 01
dauricum L. 66.590
11 02 01 09 06 11 49 1
chryseum Balf. f. & Ward 3
75.28
Lapponica
17 19 09 14 02 03
hippophaeoides Balf. f. & Ward 73.135
Lapponica
20 09 04 18 17 18 19 39 1 07
paludosum Hutch. 4 65.457
Lapponica
09 03 07 01 07
cuneatum Sm.
65.497
Lapponica
14 30 1 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 20 1 06 15
impeditum Balf. f. & W.W. Sm.
76.102
Lapponica
18 11 03 04 01 08 07 17 1 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 47 1 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 Rehder 6 64.114
Glauca
12 03 09 06 04 05 02 05 28 1
campylogynum Franchet
74.62
Camphylogyna
12 08 05 03 60 1 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.

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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.