The Effect of Light on the Tricarboxylic Acid Cycle in Green Leaves

The chlorophyll-based specific activity of cytochrome oxidase and three exclusively mitochondrial enzymes of the tricarboxylic acid cycle showed little variation between leaves of C(3) and C(4) plants or between mesophyll and bundle sheath cells of Atriplex spongiosa and Sorghum bicolor. However, a large, light-dependent transfer of label from intermediates of the tricarboxylic acid cycle to photosynthetic products was a feature of leaves of C(4) plants. This light-dependent transfer of label was barely detectable in leaves of C(3) plants and in leaves of F(1) and F(3) hybrids of Atriplex rosea (C(4)) and Atriplex patula spp hastata (C(3)). The light-dependent transfer of label to photosynthetic products in leaves of C(4) plants was inhibited by the tricarboxylic acid cycle inhibitors malonate and fluoroacetate. The requirement for continued tricarboxylic acid cycle activity was also indicated in experiments with specifically labeled succinate-(14)C. These experiments, together with the distribution of (14)C in glucose prepared from sucrose-(14)C formed during the metabolism of succinate-2,3-(14)C, confirmed that the photosynthetic metabolism of malate and aspartate derived from the tricarboxylic acid cycle, and not the refixation of respiratory CO(2), was the main path of carbon from the cycle to photosynthesis.