JARS v62n4 - New Opportunities for Breeding Fragrant Rhododendrons

New Opportunities for Breeding Fragrant Rhododendrons
Ryan N. Contreras and Thomas G. Ranney
Department of Horticultural Science
Mountain Horticultural Crops Research and Extension Center
North Carolina State University, Fletcher, NC

Two studies were recently conducted as part of a rhododendron breeding program conducted in Mills River, North Carolina, at the Mountain Horticultural Crops Research and Extension Center of North Carolina State University, with the ultimate goal of developing fragrant, evergreen, cold-hardy hybrids that are well adapted to a range of climates. These projects investigated the parentage of unusual azaleodendrons and the potential to restore fertility in sterile hybrids through the development of new allotetraploids.

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The first study was conducted to determine the parentage of selected azaleodendron hybrids. Morphological analysis historically has been used to help determine the parentage of hybrids of unknown origin; however, this can be difficult when potential parents have similar appearance, as is the case for three azaleodendron cultivars: 'Fragrans', 'Fragrans Affinity', and 'Fragrant Affinity'*. These cultivars are similar in name and appearance, and are all purported hybrids of R. catawbiense Michx. or R. ponticum L. with R. viscosum (L.) Torr. Molecular analyses were conducted to determine if the cultivars are synonyms or distinct clones and to elucidate their parental species. The three cultivars, along with related taxa in the two subgenera, Hymenanthes (Blume) K.Koch and Pentanthera (G.Don) Pojarkova, that served as the likely parents were evaluated in the study. Genetic similarity among the hybrids ranged from 53 to 71%, indicating that they are each distinct cultivars and not synonymous names for a single clone. Genetic similarity was highest between the hybrids and R. ponticum , among the evergreen rhododendrons. A dendrogram (genetic tree), generated by using the genetic similarity matrix, properly grouped the species into their respective subgenera. The three azaleodendron cultivars nested intermediately between subgenera, but more closely with subgenus Hymenanthes and particularly R. ponticum , suggesting it is the evergreen rhododendron parent. However, our results are less conclusive regarding the deciduous azalea parent. Species in the section Pentanthera often hybridize in the wild and in cultivation. The lack of clear results from the molecular analysis and the propensity for hybridization suggest that the deciduous azalea parents of these azaleodendrons may have been hybrids or not represented among the taxa included in this study.

Figure 1. Truss of 'Fragrant Affinity' 
in full flower.
Figure 1. Truss of 'Fragrant Affinity' in full flower.
R. 'Fragrant Affinity' allotetraploid 
and diploid flowers compared.
Figure 2. Flowers of allotetraploid (left) and diploid (right) 'Fragrant Affinity'.

The second study addressed the possibility of using wide hybridization to combine diverse traits that are not naturally found in a single subgenus. For example, we would like to combine the fragrance of deciduous azaleas (subgenus Pentanthera section Pentanthera ) with the more vivid flower colors and larger flower sizes of the evergreen, elepidote, rhododendrons (subgenus Hymenanthes , section Pontica ). One such hybrid with breeding potential is Rhododendron 'Fragrant Affinity'. 'Fragrant Affinity' is an azaleodendron with semi-evergreen foliage, vigorous growth, good cold hardiness (–26 °C), and fragrant, lavender flowers. This hybrid possesses unique attributes that are desirable for breeding and development of superior, cold-hardy, fragrant azaleodendrons. Unfortunately, like many other wild hybrids, 'Fragrant Affinity' is effectively sterile. Fertility in sterile, wild hybrids may often be restored by the induction of polyploidy, which can restore chromosome homology and pairing. An allotetraploid form of 'Fragrant Affinity' was successfully produced using oryzalin treatment and confirmed with flow cytometry. Fertility was restored in the allotetraploid, as confirmed by pollen staining, germination assays, and crossing studies. Pollen staining increased from 4% in the diploid to 68% in the allotetraploid and in vitro pollen germination increased from 0% to 45%, respectively. Additionally, the allotetraploid produced viable seeds and seedlings following controlled pollinations, while the diploid did not. Progeny with 'Fragrant Affinity' as a parent have been produced and are currently being evaluated for fragrance and landscape merit.

Additional information and details of this work can be found in the original articles:
First study. Contreras, R.N., T.G. Ranney, S.R. Milla-Lewis, and G.C. Yencho. 2007. Investigating parentage and hybridity of three azaleodendrons using AFLP analysis. HortScience 42(3):740-743.
Second study. Contreras, R.N., T.G. Ranney, and S.P. Tallury. 2007. Reproductive behavior of diploid and allotetraploid Rhododendron L. 'Fragrant Affinity'. HortScience 42(1):31-34.

Thanks to the Research Foundation of the American Rhododendron Society for providing critical funding to support this research.

* Name is not registered.