Cellular mechanism of reproductive barrier during cross breeding between Dendranthema grandiflorum cv. Aoyuntianshi and D. japonense.

【Objective】 Our aim is to reveal the factors resulting in low seed set during chrysanthemum breeding and seriously influencing utilization of its wild germplasm.【Method】 The interspecific distant hybridization was carried out between Dendranthema grandiflorum cv. Aoyuntianshi (female parent) and D. japonense (male parent). In addition, pollen viability of male parent was tested in culture medium just before pollination, and germination behavior of pollen grains on the stigma after pollination was examined using aniline blue staining method. In addition, embryo development after fertilization was observed with technique of paraffin section. 【Result】 Pollen viability was 10.0% just before pollination. The average number of pollen grains germinating on stigma increased slowly between 0.5 h and 8 h after pollination, and 16.1 pollen grains germinated on each stigma at 8 h after pollination. But after that, a large number of pollen grains germinated, and the number was 40.2 at 12 h after pollination. At 24 h after pollination, the number reached the highest value, 51.5. Subsequently, the number began to decrease and dropped to 42.3 at 48 h after pollination. During the process of embryo development, the percentage of normal embryos decreased gradually due to degeneration of many embryos and the values were 26.7% at stage of globular embryo, 18.3% at stage of heart embryo, 14.4% at stage of torpedo embryo, and 10.6% at stage of cotyledon embryo, respectively. Seed set in the artificial crossing was 7.6%, but the number under open pollination conditions was 51.4%. 【Conclusion】 Pollen viability of male parent and germination behavior of pollen grains on stigmas have no significant effect on seed set in the artificial crossing. And hybrid inviability, i.e. the degeneration or abortion of a large number of embryos during their developmental processes, may be the main factor leading to low seed set during cross breeding between D. grandiflorum cv. Aoyuntianshi and D. japonense.