In vivo stoichiometry of photorespiratory metabolism

Photorespiration is a major light-dependent metabolic pathway that consumes oxygen and produces carbon dioxide. In the metabolic step responsible for carbon dioxide production, two molecules of glycine (equivalent to two molecules of O₂)are converted into one molecule of serine and one molecule of CO₂. Here, we use quantitative isotopic techniques to deter-mine the stoichiometry of this reaction in sunflower leaves, and thereby the O₂/CO₂ stoichiometry of photorespiration. We find that the effective O₂/CO₂ stoichiometric coefficient at the leaf level is very close to 2 under normal photorespira-tory conditions, in line with expectations, but increases slightly at high rates of photorespiration. The net metabolic impact of this imbalance is likely to be modest.