Embryogenesis, Seed Formation and Immature Seed Germination In Vitro in Ponerorchis graminifolia Reichb. f.

Abstract

The embryogenic process of Ponerorchis graminofolia Reichb. f., a terrestrial orchid native in japan, was observed and the germination ability of its immature seeds in vitro was also studied.
1. Ovaries started elongation soon after pollination and reached full size two weeks later.
2. Ovules started to differentiated after pollination and formed a complate embryo sac with 8 nuclei on the 8th to 9th day. Double fertilization was observed in the ovules 12 to 13 days after pollination, and the fertilized polar nuclei divided into three to five endosperm nuclei.
3. The seeds increased in size rapidly, reaching a maximum days after pollination. After that time the seed coats began to shrink, then shrinkage stopped and the seeds attained final size 40 days after pollination. Embryo developed continuously during the 40 days.
4. The embryonic type at the tetrad stage was recognized as C₂ type according to Veyret′s classification, belonged to the modified F type (Coerogyne parishii type) after the tetrad stage. The main spherical part of the matured embryo was derived from the apical “ca” cells of proembryos, and the suspensor was derived from the basal “cb” cells.
5. Mature seeds of this species belong to those socalled hard to germinate. Immature seeds which developed at least beyond the octant stage began to have germination ability, and the highest germination rate (40%) was obtained about 35 to 40 days after pollination, when cultured on a Hyponex medium under light conditions.
6. A strong correlation was observed between the periods from pollination to fertilization and to embryogenesis completion, and also between the periods from pollination to fertilization and to the stages of immature seed showing the best germination rate, when examined on 47 orchid species and hycrids.