The genus Hydrangea is one of the most popular ornamental flowers because of its large and showy inflorescences. The genus comprises at least 30 taxa with centers of diversity in eastern Asia, eastern North America, and South America (Kardos et al., 2009; McClintock, 1957). This genus is further divided into two sections: Hydrangea and Cornidia. Section Cornidia consists of climbing species from tropical and subtropical areas and sect. Hydrangea contains mainly temperate species that are ornamentally attractive and commonly cultivated (Kudo et al., 2008).
Bigleaf hydrangea (H. macrophylla) is the most commonly cultivated member of the Hydrangeaceae family and is native to southern China and Japan (McClintock, 1957; Wilson, 1923). The species is cultivated as a cut flower and flowering pot plant. It is valued for its large and brightly colored inflorescences that range in color from blue to pink, unchangeably or depending on the amount of aluminum in the soil (Reed et al., 2001, 2008; Takeda et al., 1985). However, its use in areas colder than USDA hardiness zone 6 is limited by the lack of cold hardiness in the flower buds, which are formed on the previous year’s growth. Even in zones 6 and 7, late spring freezes may damage floral buds, resulting in greatly reduced flowering (Reed, 2000). Smooth hydrangea (H. arborescens) is a shrub or subshrub (up to 2–3 m tall) native to eastern North America and is hardy to zone 4. It is valued for its attractive, delicate corymbs of white flowers in early summer (Dirr, 1998). ‘Annabelle’ is a cold-tolerant smooth hydrangea cultivar that can survive in Beijing (hardiness zone 7) and other cities in northern China (zones 6 and 7).
Hybridization between distantly related species has been used in ornamental crop breeding to move desirable genes from one taxon to another (Langton, 1987). Interspecific or even intergeneric hybridizations have been made successfully in Hydrangea. Some examples are H. arborescens × H. involucrata, H. peruviana × H. serratifolia, H. scandens ssp. chinensis × H. macrophylla, H. macrophylla × H. paniculata, H. macrophylla × H. angustipetala, and H. macrophylla × Dichroa febrifuga (Jones and Reed 2006; Kardos et al., 2009; Kudo et al., 2008; Reed et al., 2001, 2008; van Gelderen and van Gelderen 2004). It seemed that seeds were produced more easily when H. macrophylla used as the maternal parent, which was also mentioned by Reed et al. (2001). Other interspecific crosses between H. macrophylla and H. arborescens or H. macrophylla and H. quercifolia failed because of postzygotic barriers. The putative hybrid seedlings died at the cotyledonary stage or the first set of true leaves stage (Reed, 2000). One effective approach to overcome postzygotic barriers that cause hydrangea seedling mortality is embryo rescue (Kudo et al., 2008). This technique has previously been used in efforts to hybridize H. macrophylla and H. arborescens. Putative hybrid plants were obtained from ovule cultures initiated from seedlings on cotyledonary stage, but the hybridity of these plants was not verified by a genetic analysis (Kudo and Niimi, 1999).
In an effort to increase the cold hardiness of H. macrophylla and expand the market into cold areas, H. arborescens was hybridized with H. macrophylla to combine the cold hardiness of H. arborescens with the wide spectrum of flower color present in H. macrophylla. The present study describes the production of interspecific hybrids between H. macrophylla ‘Blue Diamond’, ‘Schneeball’, and H. arborescens ‘Annabelle’ via ovule culture. The offspring were identified as hybrids using morphological, cytological, and SSR data.
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