Ammonia regulation of carbon metabolism in photosynthesizing leaf discs

Alfalfa (Medicago sativa L., var. El Unico) leaf discs, floating on buffer containing NH(4)Cl and photosynthesizing with (14)CO(2), produced more labeled amino acid and less sucrose than did control discs (no added NH(4)Cl). The level of pyruvate increased and that of phosphoenolpyruvate decreased. These and other changes in levels of labeled compounds led us to conclude that pyruvate kinase was activated by ammonia, resulting in increased transfer of photosynthetically incorporated carbon to synthesis of amino acid skeletons at the expense of sucrose synthesis. Carbon flow through enzymes catalyzing the anaplerotic reactions was apparently stimulated.