Investigation of the Post-fertilization Barrier and Appropriate Culture Time During the Interspecific Hybridization of Hydrangea arborescens ‘Pink Annabelle’ and H. paniculata ‘Polar Bear’

Interspecific hybridization, referring to the transfer of various traits into another species, is an important method to expand diversity in flowering plants. It is very difficult to form seeds and obtain seedlings due to various factors. Post-fertilization barriers can be overcome by embryo rescue strategies in general. The purpose of this study is to establish a protocol to improv the success of the interspecific hybridization of hydrangeas by identifying seed abortion and optimal ovule stages in the presence of post-fertilization barriers. Hydrangea arborescens and H. paniculata were crossed, and ovaries and seeds were observed every week from three weeks after pollination (WAP). Ovules were cultured in vitro from 5 WAP. As a result, it appeared that the ovaries had matured normally. Embryos in the ovary started to form between 4 and 5 WAP and developed between 5 and 7 WAP. Forty five out of 2,905 cultured ovules germinated, indicating a very low germination rate of 1.6%. Early embryos from 5 to 7 WAP, the early stages of embryo formation and development, had a low germination rate and a long germination period (average of 23.3 days). The germination rate was relatively high and the period for germination (average 11.6 days) was relatively short in cultured ovules from 8 WAP. A sharp increase in the germination period (33 days) and ovule collapse were observed at 14 WAP. In conclusion, post-fertilization barriers appeared to occur between 13 and 14 WAP, and the optimal period for the culturing of ovules was between 8 and 12 WAP for interspecific hybridization (H. arborescens x H. paniculata).