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Journal American Rhododendron Society

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Volume 41, Number 2
Spring 1987

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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.

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 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
-
pinene
-
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/-
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.

ARS Research Foundation logo

        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.


Volume 41, Number 2
Spring 1987

DLA Ejournal Home | JARS Home | Table of Contents for this issue | Search JARS and other ejournals