Rosner, A., Department of Plant Genetics, Weizmann Institute of Science, Rehovot, Israel
Porath, D., Institute for Nature Conservation Research, G.S. Wise Centre for Life Science, Tel Aviv University, Tel Aviv, Israel
Gressel, J., Department of Plant Genetics, Weizmann Institute of Science, Rehovot, Israel

Young and mature fronds of Spirodela oligorrhiza cultured in the light or dark were analyzed for their plastid ribosome distribution by electron microscopy. This distribution was correlated to the loss of integrity (hidden nicking) of the plastid heavy ribosomal RNA. In young fronds there were relatively more plastid ribosomes as well as plastid rRNA in the light than in the dark. Plastid rRNA was synthesized mainly when fronds were young. Chloroplast ribosomes disappeared from the stroma of light grown fronds upon maturation, but the relative amount of chloroplast RNA increased. In the dark the relative etioplast ribosomal density did not change upon maturation. The nicked products of the plastid heavy rRNA were detected by a modified two-dimensional gel electrophoresis. There is already some nicking of the plastid heavy rRNA at a very early stage of frond and plastid development. The rate of hidden nicking of plastid heavy rRNA was greater in the light than in the dark. The rate of nicking in fronds transferred from the dark to the light increased to the level of light grown plants at about the time of the appearance of chlorophyll. The data are discussed in the context of the relationship between plastid ribosomal RNA changes during frond maturation in light and dark grown fronds, as related to protein synthesis and of ribosome binding to thylakoids. © 1974 Oxford University Press.