A Chemical Technique Applied to the Study of Our Native Azaleas
Bruce L. King,1 Fred C. Galle,2 and Samuel B. Jones 1
1University of Georgia and 2Callaway Gardens.
A University of Georgia graduate student in plant taxonomy, Bruce L. King, has initiated a comparative study of the flavonoids of the 17 deciduous species of Rhododendron (Azaleas) native to the United States. Flavonoids are partly responsible for the flower color in Azaleas. Chemically, flavonoids are a series of C6 - C3 - C6 compounds, most of which contain a pyran ring linking the three-carbon chain with one of the benzene rings.
It has been shown that plants use flavonoids as a defense mechanism against insects and disease organisms (Levin, 1971). Numerous studies have demonstrated that flavonoid information is useful in taxonomic investigations and in the documentation and analysis of natural hybridization (Turner, 1967). Flavonoids can be used as taxonomic markers, because they possess the necessary requirements for chemical characteristics to be useful in plant taxonomy (Harborne, 1967). These requirements are: (1) structural variability; (2) chemical stability; (3) widespread distribution in the plant kingdom, and (4) easy and rapid identification. Species can often be identified by their flavonoid compounds or "fingerprints." Hybrids can usually be recognized, since they combine the chemical patterns of their two parents.
In 1971, Harborne and Williams surveyed the flavonoids of the leaves of 206 species of Rhododendron growing in gardens in England. Included in this study were five of the 17 North American species of Azaleas. Harborne and Williams found that most species of Rhododendron could be identified or else arranged in natural groupings by comparing their flavonoids.
The native azaleas of North America are a taxonomically difficult group. This taxonomic difficulty is, as Rehder (1921) suggests, due to the great variability of the species along with the existence of relatively few good 'key characters' within the group. Galle (1967) pointed out that natural hybridization is common among our native azaleas and that it has tended to blur the morphological boundaries of the species.
With the cooperation of Callaway Gardens, a grant from the Ida Cason Callaway Foundation and a Doctoral Dissertation Grant from the National Science Foundation, Mr. King has started an intensive study of the taxonomic implications of the flavonoids of our native Azaleas. Callaway Gardens has all of the native Azaleas under cultivation with the exception of Rhododendron occidentale. Mr. Fred Galle, Director of Horticulture at Callaway Gardens, began in 1954 an extensive program of hybridization among the native azaleas and has successfully produced many documented hybrids. Mr. King is in the process of determining the flavonoid profiles of the species and their documented hybrids. He is isolating the flavonoids by a combination of two-dimensional paper, column and thin layer chromatographic techniques. Identification is by the ultra violet spectrophotometric techniques. The ready availability of both the parental species and their hybrids at Callaway Gardens provides a unique opportunity for a chemotaxonomic study of a highly variable group of woody plants. Documented hybrids of woody plants are seldom available for such studies because of the length of their life cycle.
Once the flavonoid "fingerprints" are determined from the garden materials, the system will be tested in the field. Mr. King plans to determine the geographical variation, if any, in the flavonoids of several selected species. Also, he would like to document natural hybridization in one or more colonies of Azaleas where natural hybridization appears to be occurring between two species. The information obtained from these studies should provide answers to several questions: (1) What causes the problems in the identification of our native azaleas; (2) what is the population structure of the putative hybrid colonies, i.e. are the hybrids Fls, backcrosses or F3s; (3) is it possible to use a combination of flavonoid "fingerprints" and morphological characteristics to develop a better taxonomic treatment; (4) does flavonoid information provide a better understanding of the evolution of this group of shrubs? It is hoped that the information obtained from this project will provide solutions to several of the taxonomic problems in the azaleas and that it will form a basis for continued intensive studies of this interesting and highly ornamental group of species.
Mr. King's dissertation is being directed by Dr. Samuel B. Jones, Associate Professor of Botany at the University of Georgia.
Galle, F. C. 1967. Native and some introduced azaleas for southern gardens: kinds and culture. Horticultural Magazine 46:2-28.
Harbome, J. B. 1967. Comparative biochemistry of the flavonoids. Academic Press, London.
Harborne, J. B. and C. A. Williams, 1971. Leaf survey of flavonoids and simple phenols in the genus Rhododendron. Phytochemistry 10:2727-2744. Levin, D.A. 1971. Plant phenolics: an ecological perspective. Am. Nat. 105:157-181.
Rehder, A. 1921. The azaleas of North America. In: E. H. Wilson and A. Rehder. A monograph of azalea. The Cambridge University Press. pp. 107-196.
Turner, B. L. 1967. Plant chemosystematics and phylogeny. Pure and Applied Chemistry. 14:189-213.