Abstract
Reciprocal crosses between Hydrangea macrophylla f. hortensia (Lam.) Rehd. and H. arborescens L. were made to introduce useful characters of H. arborescens into H. macrophylla. The experimental results are summarized as follows. 1. Pollen grains of H. macrophylla and H. arborescens were stored at -20°C, 5°C, and 20°C with CaCl2. The pollen viability was tested by in vitro pollen germination : the pollen grains, stored at -20°C, retained germination capacity for a long term, but those stored at 5°C and 20°C lost their capacity within 5 months and 5 days, respectively ; however, when the pollen grains of both species were stored at -20°C for 11 months, over 36% of the pollens of H. macrophylla germinated, whereas none of the pollens of H. arborescens did. 2. Fresh and stored pollens of both species germinated well on the stigma in self-, intraspecific, and interspecific pollinations and the pollen tubes elongated straightforwardly to the ovary. 3. In self-and intraspecific crosses, both Hydrangea species easily set capsules and produced many mature seeds. However, the number of seed per capsule depended on cross combinations : H. macrophylla produced 10 to 54 mature seeds, whereas H. arborescens produced 19 to 31 seeds. 4. Mature seeds from self- and intraspecific crosses germinated well under in vitro and in vivo ; 58 to 85% of H. macrophylla and 14 to 52% of H. arborescens seed germinated. Pubescence was observed on the first true leaf of the latter seedlings but not on that of the former. Pubescence is an important criterion for distinguishing H. macrophylla from H. arborescens. 5. A few mature seeds were produced from 3 of 5 interspecific crosses of H. macrophylla×H. arborescens, but no seeds were produced in the reciprocal crosses. The seeds germinated in vitro and the resulting seedling showed arrested growth after the development of cotyledons, and then it died. 6. In the interspecific crosses of H. macrophylla×H. arborescens, seedlings were obtained by rescuing and culturing ovules excised 60 to 150 days after pollination, but all seedlings died after the cotyledons developed. In the reciprocal crosses, only one seedling was obtained from ovules excised 60 days after pollination but it died at the cotyledonal stage. 7. Adventitious buds formed on the cotyledons of seedlings which were obtained through ovule culture of the interspecific crosses between H. macrophylla and H. arborescens. When buds were excised and transplanted to the basal medium supplemented with 1mg/l BA, they grew into plantlets. Because the leaves on these plantlets are pubescent, they seem to be hybrids between H. macrophylla and H. arborescens.
