Germinability of embryos during seed development in Citrus (Rutaceae)

Germinability during embryo development was investigated in five Citrus cultivars with different degree of polyembryony and earliness of fruit ripening. Mature embryos extracted from fruits of the five cultivars on January 15 germinated on water by 3 days after sowing (DAS). In these cultivars, germination rates of embryos from green fruits collected in the middle of September were very low, increased rapidly in those collected in the late of September and reached 100% by 3.5 DAS in those extracted in October. The increase of germination rate was different in different cultivars. In satsuma mandarin, embryos of early ripening 'Okitsuwase' was preceded in growth and germinated about two weeks faster than those of middle ripening 'Hayashi Unshiu'. 'Hassaku' embryos showed the fastest germination rate whereas highly polyembryonic 'Seminole' embryos showed the slowest rate. It was estimated from these results that embryos matured in the early of October for 'Okitsuwase', the middle of October for 'Hayashi Unshiu' and 'Hassaku', the late of October for 'Kawano Natsudaidai' and the early of December for 'Seminole. In polyembryonic mature seeds, alone and combined applications of 1 mg/l GA₃ and 20 mg/l adenine solutions increased the number of germinated embryos per seed. Germination of the immature embryos with low germinability was enhanced by light (3000 luxes), MS medium, adenine (10-100 mg/l), gibberellin (GA₃ 1-10 mg/l) and combination of these factors. These results may contribute to the efficient cross breeding in Citrus.