Ultrastructural Changes in Cotyledons of Pineapple Guava (Myrtaceae) During Somatic Embryogenesis

Ultrastructural studies (SEM and TEM) were performed on cotyledons of pineapple guava ( Feijoa sellowianaBerg, Myrtaceae) induced to form embryos on medium containing 1.0 mg l −1(4.5μ M2,4-D) and 0.3 Msucrose. At the time of culture, the cells were filled with protein and lipid bodies. Microbodies and poorly differentiated organelles could also be seen. In contrast to germinating cotyledons, where lipid and protein reserves were quickly metabolized, cells of the embryogenically induced cotyledons showed evidence of reserve consumption only after 5 d of culture. Subepidermal cells of the upper cotyledonary surface underwent several divisions giving rise to a meristematic layer of several cells thickness from which somatic embryos developed. Embryos could also be formed directly by successive divisions of epidermal cells. Cells involved in somatic embryo formation contained a large nucleus with a conspicuous nucleolus and dense cytoplasm where numerous ribosomes, mitochondria, plastids with starch and short profiles of rough endoplasmic reticulum were present. Plasmodesmata were present both in cell walls of the meristematic cells and in few celled embryos whereas in degenerating embryos or in more advanced stages of somatic embryo development no plasmodesmata could be found. Although oil bodies were not observed in the meristematic cells they were identified in very young embryos, being the first reserve compounds to appear. Cells not involved in somatic embryo differentiation were characterized by the presence of several microbodies containing a crystalloid inclusion and elongated mitochondria.