Steady-state photosynthesis in alfalfa leaflets: effects of carbon dioxide concentration

When the CO₂ concentration to which Medicago sativa L. var. El Unico leaflets were exposed was increased from half-saturation to saturation (doubled rate of photosynthesis), glycolate and glycine production apparently decreased due to inhibition of a portion of the glycolate pathway. Serine and glycerate production was not inhibited. We conclude that serine and glycerate were made from 3-phosphoglycerate and not from glycolate and that the conversion of glycine to serine may not be the major source of photorespiratory CO₂ in alfalfa. In investigations of glycolate and photorespiratory metabolism, separate labeling data should be obtained for glycine and serine as those two amino acids may be produced from different precursors and respond differently to environmental perturbations. The increased photosynthetic rate (at saturating CO₂) resulted in greater labeling of both soluble and insoluble products. Sucrose labeling increased sharply, but there was no major shift of tracer carbon flow into sucrose relative to other metabolites. The flow of carbon from the reductive pentose phosphate cycle into the production of tricarboxylic acid cycle intermediates and amino acids increased. Only small absolute increases occurred in steady-state pool sizes of metabolites of the reductive pentose phosphate cycle at elevated CO₂, providing further evidence that the cycle is well regulated.