The protein synthesis inhibitors chloramphenicol and terramycin, and light of low intensity were used to retard the rate of chlorophyll formation in illuminated dark grown pea leaves. In the control leaves the onset of photosynthesis, as measured by carbon dioxide exchange of the whole leaves, and reduction of ferricyanide and metmyoglobin and photo-oxidation of ascorbate in isolated chloroplasts, was observed after 2–4 hours illumination. The photosynthetic activity of the treated leaves did not commence until 10–12 hours illumination had elapsed. In both the control and treated leaves the onset of photosynthesis occurred when the total chlorophyll content was 0.04 mg/g fresh weight. The precise point of photosynthetic inception was apparently more related to the attainment of a specific total chlorophyl content than to the ratio of chlorophyllato chlorophyllb. A marked increase in the evolution of carbon dioxide in the light was observed in the treated leaves during the first 10 hours of greening. This observation could not be ascribed to photorespiration since the leaves did not possess an active photosystem. It is suggested that the enhanced respiration may have been due to the light-induced activation of synthetic pathways responsible for the formation of chloroplast constitue
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