Carbon Dioxide Release During Photosynthesis: Connecting Gas Exchange Behavior With Biochemistry.

During photosynthesis, CO₂ uptake is counterbalanced by concurrent CO₂-releasing processes, complicating the interpretation of gas exchange measurements. While photorespiration accounts for a significant portion of this CO₂ release, emerging evidence indicates that there are additional metabolic pathways that release CO2 during photosynthesis. This metabolism-termed day respiration (often Rd) or respiration in the light (RL)-is now recognized as an independent and significant source of CO2 emission during photosynthesis. Here we revisit classical models of photosynthesis and incorporate new insights from isotopic labeling and metabolic flux analysis (MFA) to investigate the biochemical basis of RL. We identified the cytosolic glucose-6-phosphate (G6P) shunt through the oxidative pentose phosphate pathway (OPPP) as the predominant contributor to RL. This shunt explains some long-standing anomalies in Calvin-Benson-Bassham (CBB) cycle labeling. Under non-stressed conditions, RL remains stable across varying CO₂ concentrations and light intensities. Under heat stress, RL shifts toward a plastidial source. Together, these findings resolve longstanding questions about carbon flux during photosynthesis and improve our understanding of RL by explaining its metabolic origin, physiological significance in carbon balance during photosynthesis, and regulation under varying environmental conditions.