Relationship between Photosynthesis and Protein Synthesis in Maize: II. Interconversions of the Photoassimilated Carbon in the Ear Leaf and in the Intermediary Organs to Synthesize the Seed Storage Proteins and Starch

The mechanisms priming the production, the movement, and the transient and final storage of the photoassimilated carbon in the maize plant were examined at the metabolic level during the formation of the seed, with the ultimate aim to identify metabolic steps restricting grain yield and explaining the delay of formation of the reserve molecules. Under normal field conditions, we show that maize directly supplies the developing seed with the photoassimilated carbon which undergoes numerous interconversions from the ear leaf to the grain. The proteins, either in the leaf or in the seed, are primarily synthesized from incoming amino acids. Nevertheless, a secondary in situ synthesis of amino acids provides the proteins with new amino acids. The amino acids of this second set, slowly synthesized in the seed from the photosynthetic carbon skeletons, are not detected in their free form but immediately and regularly incorporated into the seed proteins, in such a way that, after 4 days of chase, the proportion of the radioactive labeling of the amino acids of the different storage protein groups corresponds to their amino acid composition. In the leaf, the labeling of proteins also arises from different metabolisms, but mainly from the photosynthetic metabolism. Contrary to the seed proteins, the time course of the labeled leaf proteins implies a rapid turnover. The second labeling of starch and proteins in the ear leaf involves a reassimilation of CO₂, a process optimizing the carbon uptake in maize.