Light and electron microscopic examination of the embryo and endosperm development in the natural tetraploid Trifolium pratense L.

The ultrastructure of the embryo cells in ovules, from fertilization to the embryo maturity stage in the natural tetraploid Trifolium pratense L. that has a very low rate of seed formation, was examined. Following fertilization the vacuolar organization in the zygote changes. The zygote was a polarized cell and contained a central nucleus, mitochondria, plastids, ribosomes, and lipid bodies. Ribosomal concentration increases significantly after fertilization. The first division of the zygote was transverse or oblique and unequal. The primary endosperm nucleus divides before the zygote nucleus, forming a coenocytic nuclear endosperm; however, part of it later becomes cellular. At the earliest stage of embryo development, the cells were vacuolate, and plastids and mitochondria were simple in structure. During all stages of embryogenesis the suspensor cells were less electron dense than the adjoining embryo cells. Endosperm cellularization begins when the embryo has developed the globular embryo proper. Cellularization starts at the micropylar end of the embryo sac and progresses toward the chalazal end. Dictyosome activity, ribosomal aggregation, and the amount of rough endoplasmic reticulum were highest during the late globular embryo stage. In addition, the vacuolar volume in the cells was reduced. Lipid bodies were present up to the early globular stage, then disappeared. The inner cell walls of the embryo were thin, with many plasmodesmata. These walls begin to thicken at the late globular stage. The results show a corresponding increase in the amount and activity of the metabolic machinery as the development of the embryo progresses.