Rhododendrons and azaleas (Rhododendron L.) are among the most widely grown ornamental plants in the world. There are over 1000 species recognized in eight subgenera (Chamberlain et al., 1996). Species within subgenera readily hybridize and have resulted in thousands of cultivars (Väinölä, 2000). Although intrasubgeneric hybridization is responsible for the majority of existing cultivars, species in different subgenera are sometimes capable of hybridizing.
Azaleodendrons are hybrids between deciduous azaleas (subgenus Pentanthera (G.Don) Pojarkova) and nonscaly leaved rhododendrons (subgenus Hymenanthes (Blume) K.Koch) and constitute some of the oldest hybrids within the genus. The first interspecific hybrid rhododendron reported was ‘Azaleoides’ resulting from a chance cross between Rhododendron periclymenoides (Michx.) Shinners and R. ponticum L. in London circa 1820 (Hillier Nurseries, 2002). Such hybrids have the potential to combine the fragrance of the deciduous azaleas with darker flower colors, larger flower size, and persistent foliage of evergreen rhododendrons. One such hybrid with breeding potential is Rhododendron ‘Fragrant Affinity’. ‘Fragrant Affinity’ is an azaleodendron with semievergreen foliage, vigorous growth, good cold hardiness (–26 °C), and fragrant, lavender flowers (personal observations). This intersubgeneric hybrid, believed to be a cross between R. ponticum and R. viscosum (L.) Torrey (Contreras, 2006), possesses unique attributes that are desirable for breeding and development of superior, cold-hardy, fragrant azaleodendrons. Unfortunately, like many other wide hybrids, it appears to be sterile.
Hybrid sterility, referred to as chromosomal sterility or chromosomal hybrid sterility, is often the result of improper chromosome pairing during gametogenesis resulting from structural differences in parental chromosomes. This results in meiotic abnormalities such as univalents and lagging chromosomes; however, other mechanisms may also be involved in hybrid sterility (Lu and Bridgen, 1997). In a study using Alstroemeria aurea Graham × A. caryophyllaea Jacq., Lu and Bridgen (1997) determined that its sterility resulted from complex fertility/sterility-regulating mechanisms and was not simply the result of parental chromosome differences. In cases in which sterility is incited by improper chromosome pairing, doubling the chromosome complement (polyploidization) of sterile hybrids to produce allotetraploids provides a homolog for each chromosome to pair with during meiosis and can allow for the development of fertile gametes (Hadley and Openshaw, 1980; Lu and Bridgen, 1997; Stebbins, 1950; van Tuyl and De Jeu, 1997; Zadoo et al., 1975).
Natural polyploids exist in the genus Rhododendron, including triploids (2n = 3x = 39), tetraploids (2n = 4x = 52), hexaploids (2n = 6x = 78), octaploids (2n = 8x = 104), and dodecaploids (2n = 12x = 156) (Ammal, 1950; Ammal et al., 1950). Artificial polyploid rhododendrons have also been developed to increase ornamental characteristics such as flower size and texture, extend time of flowering, produce more compact plants, and facilitate crosses not possible at the diploid level (Eiselein, 1994; Kehr, 1996a, 1996b; Pryor and Frazier, 1968; Tolstead and Glencoe, 1991; Väinölä, 2000). Polyploid rhododendrons have been induced with various chemical doubling agents, including colchicine (Kehr, 1996a) and oryzalin (Väinölä, 2000).
The objective of this study was to evaluate fertility of diploid and allotetraploid forms of R. ‘Fragrant Affinity’ and to determine if induced polyploidy is an effective method for restoring fertility in hybrids of distantly related rhododendrons.
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