The Mechanism of Polyembryonic Seed Set in Paspalum notatum

Four facultatively apomictic varieties of bahia grass (Paspalum notatum Flugge) were studied for making it clear the mechanism of multiple embryo seed set development in polyembryonic ovules. Over 300 ovaries at and after anthesis were collected from each variety, treated with Herr's clearing fluid and observed with Nomarski differential interference-contrast optics. At anthesis, the formed typical embryo sac located in micropylar end (first sac) were 92.5 to 100%, and those in the other ends (other sacs) were 40.4 to 86.0% among the 4 varieties. After anthesis, the first sac divided dominantly when compared with the other sacs. At 4 d after anthesis, the rates of the ovules contained the developed first sacs were 56 to 87% comparable to 0 to 1% of the other sacs among the 4 varieties. However, 4 to 17% of the other sacs also showed embryo formations but endosperm. In final, the first sac developed well and occupied the whole space of the ovule, and the embryos formed in the other sacs coexisted with the first sac. From the above results, it can be concluded that the first sac has temporal dominant in embryo sac formation, and has the positional dominant in fertilization and subsequent development of embryo sac when compared with the other sacs in polyembryonic ovules. The egg cell of the other sac (2n) located close to the endosperm of micropylar sac, divided with stimulation of the first sac development, and finally also formed seed-forming embryo. That is why twin, thrice or four seedlings were observed when in germination. The estimation of degree of sexual or apomixis was also discussed based on the observations.