CHARACTERIZATION OF SEED AND EMBRYO ABORTION DURING FRUIT DEVELOPMENT IN CITRUS CULTIVARS POLLINATED BY 'NISHIUCHI KONATSU' (CITRUS TAMURANA) AND A PRELIMINARY TRIAL OF EMBRYO RESCUE IN ABORTING EMBRYOS

‘Hyuganatsu’ (HY) is a self-incompatible citrus cultivar grown in specific prefectures in Japan. However, its bud mutation, ‘Nishiuchi Konatsu’ (NK), is self-compatible, although the majority of seeds are aborted in mature fruits. In our previous studies, we revealed that NK produces a proportion of unreduced 2n pollen grains, resulting in unusual seed development. In this study, we therefore focused on seed and embryo development after pollination of HY, NK, and ‘Hassaku’ (HS), which is compatible with both HY and NK, with NK pollen. Seeds from HY × NK and NK × NK fruits were significantly smaller than those from HS-pollinated fruits (i.e. HY × HS and NK × HS) at 10-12 weeks after pollination, suggesting that the seed abortion phenotype was expressed during this time period. Moreover, embryos that formed from NK self-pollination were shown to have aborted at 10 weeks after pollination, whereas control embryos reached the globular or heart-shaped stage. HS × NK resulted in both normal and aborted seeds present in the fruit. Development of some embryos was found to cease at 12 weeks after pollination. At 14 weeks after pollination, normal and aborted seeds were easily distinguishable. Preliminary attempts of embryo rescue were also conducted in order to recover triploid seedlings. Aborting embryos obtained from HY × NK and NK × NK were placed on MT media supplemented with 500 mg/L malt extract, 50 g/L sucrose, and 5 mM GA₃. As a result, several triploid plantlets were confirmed by flow cytometry after embryo recovery from both crosses, although the success rate was not high.