After the transfer of 14-day old dark-grown bean seedlings into continuous illumination the etioplasts of the primary leaves develop into chloroplasts in a synchronous manner. Levels of activity in the leaves of most of the enzymes which have been implicated in the photosynthetic carbon cycle have been determined during the first 45 hours of chloroplast development. All of the enzymes investigated were present in etiolated leaves and the activities of most of them increased in response to illumination. One of the early effects of illumination was a rise in the protein content of the leaves, the level of which continued to rise throughout the 45 hours of illumination. Increases in the activity of some of the enzymes, notably phosphoglycerate kinase, fructosediphosphate aldolase, transketolase, NAD-linked triosephosphate dehydrogenase and triosephosphate isomerase, appeared to begin soon after the commencement of illumination. The onset of light-induced increases in the activities of ribosephosphate isomerase, phosphori-bulokinase and ribulosediphosphate carboxylase, showed lag periods of similar duration to the ‘positive’ photoresponse described by Mohr (1966). The longer lag period before an increase of NADP-Iinked triosephosphate dehydrogenase occurred is consistent with the suggestion of Ziegler and Ziegler (1965) that photosynthetically formed ATP and NADPH may be essential prerequisites for the synthesis of this enzyme. Possible mechanisms of control of the enzyme levels are discussed.

The data do not provide any evidence in support of the participation of phosphopyruvate carboxylase, hexosediphosphatase and transaldolase in an important role in connection with the photosynthetic carbon cycle of bean leaves.