Abstract
For breeding Hydrangea, it is an advantage to breed new hybrid cultivars in as short a time as possible. To this end, we used ovule culture and regulation of flowering to shorten the breeding period of hybrids produced from crosses between Hydrangea serrata (Thunb.) Ser. and H. macrophylla (Thunb.) Ser. Interspecific crosses between H. serrata and H. macrophylla were carried out in June. One year after crossing, hybrid plants were induced to flower by cool temperature treatments of 15°C for 60 days (under ambient daylight). Dormancy of hybrid plants was broken by low temperature treatment at 5°C for 50 days. Hybrid plants flowered in December, 18 months after crossing. The flowering rates among the four hybrid combinations ranged from 77 to 96%. Interspecific hybrid ‘04MaP1’ which flowered in December was crossed with H. macrophylla ‘Sumida no hanabi’. Sixty days after crossing, ovules were excised from capsules and cultured on 1/2 MS medium. Germinated seedlings were raised in a culture/acclimation room at 25°C with a 16-hour light/8-hour dark photoperiod. These hybrid plants were transferred into a greenhouse without temperature regulation in July (7 months after crossing), and flowered the following April or May (18 months after crossing). Using these methods, we obtained flowering Hydrangea hybrids at the second generation, 3 years after the first crossing.
