Hydrangea was systematically described by McClintock (1957). She included 23 species with a disjunct distribution in both temperate and tropical regions of eastern Asia, eastern North America, and South America. Hydrangea macrophylla is the most popular of these species, and it is one of the most commercially important flowering shrubs grown worldwide. Hydrangea macrophylla is native to southern China and Japan and was cultivated there long before introduction into Europe in the 1800s (McClintock, 1957; Wilson, 1923).
The genetic diversity among H. macrophylla cultivars is limited as a result of the restricted native distribution and multiple breeding programs that used the same taxa and had similar breeding goals (Haworth-Booth, 1984; van Gelderen and van Gelderen, 2004). Most of the cultivars in existence today are derived from plants bred in the early 20th century through controlled crosses, open pollinations, or branch sports from introductions of wild-collected germplasm in the 19th and 20th centuries (Haworth-Booth, 1984; McClintock, 1957). Although over 1000 cultivars of H. macrophylla exist, many of them are similar in growth habit, floral characteristics, and disease susceptibility (Dirr, 2002). Recently, the introduction of remontant flowering or reblooming cultivars such as ‘Bailmer’ (Endless Summer® The Original) has increased the presence of hydrangeas in American commerce and gardens. New sources of genetic diversity are needed to develop cultivars with improved disease resistance, ease of production, and improved garden performance. Dan Hinkley, former owner of Heronswood Nursery, Bleddyn and Sue Wynn-Jones, owners of Crûg Farm Plants, and Scott McMahan, owner of McMahan's Nursery, have recently introduced new wild-collected H. macrophylla germplasm (personal communication).
Although interspecific and intergeneric hybridizations have been attempted within the Hydrangeaceae, most of the resultant hybrids were weak, sterile, or had reduced fertility and were of no commercial value. Hybridizations of H. macrophylla with H. anomala D. Don ssp. petiolaris (Siebold & Zuccarini) McClintock (Haworth-Booth, 1984), H. arborescens Linnaeus (Kudo and Niimi, 1999; Reed, 2000), H. paniculata Siebold (Reed, 2004; Reed et al., 2001), H. quercifolia Bartram (Kudo et al., 2002; Reed, 2000), H. serrata (Thunberg) Seringe (Dirr, 2004; Zonneveld, 2004), and Dichroa febrifuga Loureiro (Jones et al., 2006; Kardos et al., 2006; Reed et al., 2008) have been reported. In addition, a preliminary report of hybridization between H. macrophylla and H. angustipetala has been published (Kardos et al., 2006), but the report lacks details on procedures and description of the hybrids. A hybrid with commercial potential was produced through ovule culture from the cross H. scandens ssp. chinensis to H. macrophylla, although the hybrid was sterile (Kudo et al., 2008). Unlike most of the interspecific hybrids, the intergeneric hybrids from D. febrifuga × H. macrophylla are vigorous, fertile, and show potential for further breeding and/or introduction (Reed et al., 2008). Additional interspecific hybrids have been produced from H. arborescens ‘Dardom’ × H. involucrata Siebold (Jones and Reed, 2006) and H. involucrata × H. aspera D. Don (Dirr, 2004).
Rinehart et al. (2006) using microsatellite [simple sequence repeat (SSR)] markers showed a close relationship among H. macrophylla, H. scandens ssp. chinensis (H. angustipetala), and D. febrifuga. Jones et al. (2006), Kardos et al. (2006), and Reed et al. (2008) have produced D. febrifuga × H. macrophylla hybrids, confirming the affinities revealed by the SSRs. Hydrangea macrophylla and H. angustipetala accessions were found to be diploid with 2n = 2x = 36 chromosomes (Cerbah et al., 2001), although triploid H. macrophylla cultivars have been identified (Jones et al., 2007; Zonneveld, 2004). Zonneveld (2004) reported nuclear DNA contents of 4.54 and 4.76 pg for H. macrophylla and H. angustipetala, respectively. The same ploidy level, similar nuclear DNA contents, and a high degree of relatedness between H. macrophylla and H. angustipetala, as indicated by SSR data, suggest the potential for successful interspecific hybridization.
Hydrangea angustipetala is a source of genetic diversity for traits such as powdery mildew resistance, early flowering, and narrow, evergreen foliage for incorporation into new hybrids with H. macrophylla cultivars (personal observation). Hydrangea macrophylla, native to southern China and Japan, characteristically possesses 10 to 20 cm long, 6 to 14 cm wide, coarsely toothed, matte green to lustrous dark green leaves, stout stems, and lacecap or mophead inflorescences 8 to 25 cm in diameter on 1 to 2 m high and wide plants. Flower color in H. macrophylla ranges from white to pink to purple to blue. Hydrangea angustipetala, native to Japan, China, and Taiwan, is deciduous to evergreen, flowers ≈4 weeks earlier than H. macrophylla, and displays resistance to powdery mildew (personal observation). Mature plant size is 1.5 m high and wide with pubescent, dentate, shiny dark green leaves ≈6 cm long and 2.5 cm wide. Inflorescences are lacecap, ≈7.5 cm in diameter consisting of cream-yellow to white, fragrant fertile flowers surrounded by a few sterile flowers with three or four white sepals per flower. Flowers develop at each node, often the entire length of the stems. Variation exists within this species for growth habit, size of foliage, degree of foliage retention in winter, cold-hardiness, inflorescence size, and fragrance (personal observation). Hybridization between this species and H. macrophylla could result in hybrids with narrow, semievergreen to evergreen, lustrous foliage, improved powdery mildew resistance, early flowering, and fragrant flowers.
The taxonomy of H. angustipetala is debatable. Hydrangea angustipetala is listed as H. scandens ssp. angustipetala Mallet (Mallet, 1994), H. scandens ssp. chinensis (Maximowicz) McClintock (McClintock, 1957), and H. scandens ssp. chinensis f. angustipetala Hayata (Haworth-Booth, 1984; Zonneveld, 2004). Zonneveld (2004) suggests that H. angustipetala should be a separate species from H. scandens (L. f.) Seringe based on observations of heterogeneous DNA content in H. scandens (4.16 pg), H. scandens ssp. chinensis f. angustipetala (4.72 pg), and H. scandens ssp. chinensis f. liukiuensis (Nakai) McClintock (4.02 pg). Genome size differences are supported by SSR data, which also suggest that H. scandens ssp. chinensis f. angustipetala is genetically distinct from H. scandens ssp. chinensis f. liukiuensis (Rinehart et al., 2006). Therefore, the parental material used in this study is treated herein as H. angustipetala, although it could also be labeled H. scandens ssp. chinensis f. angustipetala.
The objective of this study was to hybridize, verify, and describe hybrids between H. macrophylla and H. angustipetala. The long-term goal of the research is to develop plants exhibiting a combination of desirable traits that have commercial value.
Adkins, J.A. & Dirr, M.A. 2003 Remontant flowering potential of ten Hydrangea macrophylla (Thunb.) Ser. cultivars HortScience 38 1337 1340
Cerbah, M., Mortreau, E., Brown, S., Siljak-Yakovlev, S., Bertrand, H. & Lambert, C. 2001 Genome size variation and species relationships in the genus Hydrangea Theor. Appl. Genet. 103 45 51
Heslop-Harrison, J. & Heslop-Harrison, Y. 1970 Evaluation of pollen viability by enzymatically induced fluorescence; intracellular hydrolysis of fluorescein diacetate Stain Technol. 45 115 120
Jin, L. & Chakraborty, R. 1994 Estimation of genetic distance and coefficient of gene diversity from single-probe multilocus DNA fingerprinting data Mol. Biol. Evol. 11 120 127
Jones, K.D. & Reed, S.M. 2006 Production and verification of Hydrangea arborescens ‘Dardom’ × H. involucrata hybrids HortScience 41 564 566
Jones, K.D., Reed, S.M. & Rinehart, T.A. 2006 Wide crosses in the Hydrangeaceae: Dichroa febrifuga × Hydrangea macrophylla Proc. Southern Nursery Assn. Res. Conf. 51 577 579
Jones, K.D., Reed, S.M. & Rinehart, T.A. 2007 Analysis of ploidy level and its effects on guard cell length, pollen diameter and fertility in Hydrangea macrophylla HortScience 42 483 488
Kardos, J.H. 2008 Interspecific and intergeneric hybridization involving Hydrangea macrophylla (Thunberg) Seringe and inheritance studies in H. macrophylla PhD Diss. University of Georgia
Kardos, J.H., Robacker, C.D., Dirr, M.A. & Rinehart, T.A. 2006 Production and verification of hybrids from Hydrangea macrophylla × H. angustipetala and H. macrophylla × Dichroa febrifuga Proc. Southern Nursery Assn. Res. Conf. 51 570 572
Kudo, N., Kimura, Y. & Niimi, Y. 2002 Production of interspecific hybrid plants by crossing Hydrangea macrophylla f. hortensia (Lam.) Rehd. and H. quercifolia Bartr. through ovule culture Hort. Res. Japan 1 9 12
Kudo, N., Matsui, T. & Okada, T. 2008 A novel interspecific hybrid plant between Hydrangea scandens ssp. chinensis and H. macrophylla via ovule culture Plant Biotechnol. 25 529 533
Kudo, N. & Niimi, Y. 1999 Production of interspecific hybrids between Hydrangea macrophylla f. hortensia (Lam.) Rehd. and H. arborescens L J. Jpn. Soc. Hort. Sci. 68 428 439
Reed, S.M. 2004 Floral characteristics of a Hydrangea macrophylla × H. paniculata hybrid Proc. Southern Nursery Assn. Res. Conf. 49 580 582
Reed, S.M., Jones, K.D. & Rinehart, T.A. 2008 Production and characterization of intergeneric hybrids between Dichroa febrifuga and Hydrangea macrophylla J. Amer. Soc. Hort. Sci. 133 84 91
Reed, S.M., Riedel, G.L. & Pooler, M.R. 2001 Verification and establishment of Hydrangea macrophylla ‘Kardinal’ × H. paniculata ‘Brussels Lace’ interspecific hybrids J. Environ. Hort. 19 85 88
Rinehart, T.A., Scheffler, B.E. & Reed, S.M. 2006 Genetic diversity estimates for the genus Hydrangea and development of a molecular key based on SSR J. Amer. Soc. Hort. Sci. 131 787 797
Zonneveld, B.J.M. 2004 Genome size in Hydrangea Van Gelderen C.J. & van Gelderen D.M. Encyclopedia of hydrangeas Timber Press Portland, OR