Hydrangea macrophylla is widely grown as both a landscape and florist plant. In the landscape, it is valued for its large, long-lasting inflorescences that are produced in early summer. As a florist's plant, it is usually forced into bloom for Easter and Mother's Day sales (Bailey, 1989). Although only H. macrophylla ssp. macrophylla is used in the florist's trade, both H. macrophylla ssp. macrophylla (bigleaf hydrangea) and H. macrophylla ssp. serrata (mountain hydrangea) are sold as garden plants. The two subspecies are differentiated by the larger leaves and inflorescences of H. macrophylla ssp. macrophylla (McClintock, 1957). Interest in the species has increased in recent years as a result of the introduction of cultivars with new inflorescence forms and color patterns from Europe and Japan and the identification of remontant, or reflowering, cultivars (Dirr, 2004). Breeding programs in the United States are currently focusing on developing H. macrophylla cultivars that combine remontant ability with cold hardiness, pest resistance, and ornamental traits.
Although hundreds of H. macrophylla cultivars were bred in Europe in the early 20th century (Bertrand, 2001; Haworth-Booth, 1984; van Gelderen and van Gelderen, 2004), little scientific literature exists related to breeding and genetics of this or related species. In particular, only a few cytological studies have been conducted in Hydrangea L. Chromosome number within the genus ranges from 2n = 2x = 30 in H. involucrata Sieb. to 2n = 4x = 72 or 2n = 6x = 108 in H. paniculata Sieb. (Funamoto and Tanaka, 1988). Asiatic species have significantly larger genomes than North and South American species (Cerbah et al., 2001; Zonneveld, 2004). For example, the Asiatic species H. involucrata (2n = 2x = 30) has approximately twice the nuclear DNA content of the American species H. arborescens (2n = 2x = 38) although both are diploid species (Zonneveld, 2004). A considerable difference in chromosome size between these two species was strikingly apparent in their hybrid (Jones and Reed, 2006).
A somatic chromosome number of 2n = 2x = 36 was first reported for H. macrophylla in 1957 (Haworth-Booth, 1984). Funamoto and Tanaka (1988) confirmed this number using H. macrophylla ssp. serrata plants collected from five locations. Kudo and Niimi (1999) reported 52 chromosomes in H. macrophylla ssp. macrophylla ‘Blaumeise’ [syn. ‘Blue Sky’ (Bertrand, 2001)].
DNA content of H. macrophylla has been investigated using flow cytometry. Working with 4′,6-diamidino-2-phenylidole (DAPI)-stained nuclei, Demilly et al. (2000) found 100 diploid and 21 triploid H. macrophylla ssp. macrophylla cultivars. All 23 H. macrophylla ssp. serrata taxa examined were diploids. Unfortunately, these authors did not list names of any cultivar evaluated in this study. They reported that flow cytometric results were supported by chromosome counts with diploids having 36 and triploids having 54 chromosomes, but no details as to cultivars examined or number of cells counted were provided. Flow cytometric measurements of nuclear DNA content of five H. macrophylla ssp. macrophylla and five H. macrophylla ssp. serrata plants were made using DNA intercalating, GC-specific and AT-specific fluorochromes (Cerbah et al., 2001). The H. macrophylla ssp. macrophylla plants had 11.7% more nuclear DNA than the H. macrophylla ssp. serrata plants. Both subspecies had similar GC percentages of 41.0 and 40.6%, respectively. Chromosome counts were also made from root tips. All plants examined had 36 chromosomes. Plants included in this study were identified by accession number rather than cultivar name.
The most recent study of ploidy differences within H. macrophylla was reported by Zonneveld (2004). Propidium iodide-stained nuclei of 25 H. macrophylla ssp. macrophylla and 18 H. macrophylla ssp. serrata taxa were examined using flow cytometry. All of the H. macrophylla ssp. serrata and 16 of the H. macrophylla ssp. macrophylla plants were found to be diploids. The remaining H. macrophylla ssp. macrophylla plants were identified as triploids. Although cytological data were not presented, the author stated that triploidy was verified from chromosome counts of root tip cells. The H. macrophylla ssp. serrata taxa had ≈5.8% less nuclear DNA than the diploid H. macrophylla ssp. macrophylla cultivars.
Information on ploidy level of specific H. macrophylla cultivars will help in selecting parents for both intra- and interspecific hybridizations. The objective of this study was to evaluate ploidy in a diverse group of H. macrophylla cultivars. Ploidy was first estimated using flow cytometry and then confirmed in selected diploid and triploid plants using chromosome counts. The potential for using stomatal guard cell length and pollen grain size to identify ploidy level of H. macrophylla plants was also examined. Finally, fertility of triploid cultivars was estimated through pollen staining and controlled pollinations.
Bretagnolle, F. & Thompson, J.D. 1995 Gametes with the somatic chromosome number: Mechanisms of their formation and role in the evolution of autopolyploid plants New Phytol. 129 1 22
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
Kudo, N. & Niimi, Y. 1999 Production of interspecific hybrids between Hydrangea macrophylla f. hortensia (Lam.) Rehd. and H. arborescens L Jpn. Soc. Hort. Sci. 68 428 439
Nagl, W. 1978 Endopolyploidy and polyteny in differentiation and evolution: Towards an understanding of quantitative and qualitative variation of nuclear DNA in ontogeny and phylogeny North-Holland Publishing N.Y
Quinn, A.A., Mok, D.W.S. & Peloquin, S.J. 1974 Distribution and significance of diplandroids among the diploid Solanums Am. Potato J. 51 16 21
Reeves, A. and J. Tear. 2000. MicroMeasure for Windows, version 3.3. 22 Mar. 2007. Free program distributed by the authors over the Internet from http://www.colostate.edu/Depts/Biology/MicroMeasure.
Zonneveld, B.J.M. 2004 Genome size in Hydrangea 245 251 van Gelderen C.J. & van Gelderen D.M. Encyclopedia of hydrangeas Timber Press Portland, Ore