Ultrastructure of chloroplast protuberances in rice leaves preserved by high-pressure freezing

High-pressure freezing and freeze substitution were used to prepare leaves of rice (Oryza sativa L.) for ultrastructural analysis. Under these prepara tive conditions, plastid-derived stroma-containing protuberances were prese rved and described with the electron microscope for the first time. Similar protuberances were observed previously only in living cells examined with the light microscope. Infoldings of the chloroplast inner envelope were a p rominent ultrastructural feature of protuberances. Infoldings were also obs erved in the main body of the chloroplasts and sometimes appeared contiguou s with thylakoid membranes. Protuberances also contained infoldings in the form of bifurcated tubules. Apparent interconnection between protuberances of adjacent plastids was observed only in one instance. A distinct gradient in the staining density of thylakoid lumina appeared to be a function of g rana position and orientation relative to the cell wall. Immunocytochemistr y was used to determine that the stroma within protuberances contained 1,5- bisphosphate carboxylase/oxygenase enzyme.