Effect of light intensity on the distribution of C 14 in sunflower leaf metabolites during photosynthesis

Abstract

1. 1. Samples of sucrose, alanine, and malic acid obtained from sunflower leaves which had photosynthesized C 14O 2 under strong and weak light were degraded, and the C 14 content of the individual carbon atoms was determined. 2. 2. Under strong and weak light conditions, the distribution of C 14 in alanine and the glucose or fructose of sucrose (carbon atoms 1, 2, 3, and 4, 5, 6) is similar. 3. 3. Under weak-light photosynthesis, the distribution of C 14 in alanine and in the three carbon atoms of malic acid, β-carboxyl excluded, is similar; however, during photosynthesis in strong light, these three carbon atoms of malic acid were less uniformly labeled than those of alanine. 4. 4. This dissimilarity in labeling between alanine and these three carbon atoms of malic acid during strong- and weak-light photosynthesis was interpreted as a block in the conversion of pyruvic acid to malic acid via the tricarboxylic acid cycle caused by the high light intensity.