Ultrastructural and histochemical postfertilization megagametophyte and zygotic embryo development of white spruce (Picea glauca) emphasizing the deposition of seed storage products

Abstract

Deposition of major storage substances in the megagametophyte and embryo of white spruce (Picea glauca (Moench) Voss) was studied ultrastructurally and histochemically during seed development. Lipid bodies appeared to be secreted by the smooth endoplasmic reticulum. In the megagametophyte they were deposited rapidly from the club-shaped embryo stage until the early organogenesis of the embryo. Major lipid accumulation in the embryo took place during rapid cotyledon development and simultaneously with the buildup of protein bodies. Formation of protein bodies in the megagametophyte and in the embryo was first detected approximately 6 and 29 days after fertilization, respectively. It is suggested that in the megagametophyte, this process proceeded by (i) deposition of amorphous protein clumps onto tonoplasts of subdividing vacuoles during early stages of protein body formation, (ii) fusion of small cytoplasmic vesicles possibly derived from the rough endoplasmic reticulum, and (iii) deposition of protein around dense, membrane-bound vesicles attached to tonoplasts. The third process was not observed in the embryo. During advanced formation, fusion of cytoplasmic vesicles into developing protein bodies was the only process observed in the megagametophyte and in the embryo. White spruce seed protein bodies contained a variable number of crystalloids and globoid cavities except in the embryo in which only single globoid cavities were observed. Key words: Picea, ultrastructure, histochemistry, megagametophyte, embryo, seed.