A histochemical and ultrastructural study of the ontogeny and differentiation of pollen inPodocarpus macrophyllus D. Don

Abstract

Male cones ofPodocarpus macrophyllus D. Don enter a period of dormancy lasting almost a year after the differentiation of archesporial tissue. The cell walls of the sporogenous and tapetal cells are different in composition from those of the cells comprising the wall of the microsporangium. The walls of tapetal cells undergo complete dissolution but the naked protoplasts do not invade the cavity of the microsporangium, and eventually degeneratein situ. Sporopollenin-containing bodies are formed on the tapetal plasmalemma although no specific tapetal organelles can be singled out as sites of synthesis of sporopollenin precursors. The original walls of the microspore mother cells are broken down completely and replaced by a thin callose-like wall. No cytomictic channels are formed prior to or during early meiosis. The outer nuclear membrane of the sporogenous cells forms numerous vesicles which likely play an important role in preparing the cell for meiosis and in the breakdown of the original sporogenous cell wall and the formation of the new wall. Pronounced evaginations and invaginations of the nuclear envelope during the tetrad stage are seen which again indicate vital nucleo-cytoplasmic exchange at the time when species specific sexine layer is being laid down. The microspore protoplast synthesizes a portion of sporopollenin precursors. Sexine and part of nexine I are laid down during the tetrad stage on lamellae of unit membrane dimensions while nexines II and III are formed after the dissolution of the tetrads by the coalescence of small, electron dense particles. Cells of the male gametophyte are initially separated from each other by distinct cell walls often traversed by plasmodesmata. Mature pollen grains have appreciable reserves of protein, lipid and starch. Results of histochemical and scanning electron microscopical observations are also reported and discussed.