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 CO 2 during photosynthesis. This metabolism—termed day respiration (often R d ) or respiration in the light ( R L )—is now recognized as an independent and significant source of CO 2 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 R L . We identified the cytosolic glucose‐6‐phosphate (G6P) shunt through the oxidative pentose phosphate pathway (OPPP) as the predominant contributor to R L . This shunt explains some long‐standing anomalies in Calvin‐Benson‐Bassham (CBB) cycle labeling. Under non‐stressed conditions, R L remains stable across varying CO₂ concentrations and light intensities. Under heat stress, R L shifts toward a plastidial source. Together, these findings resolve longstanding questions about carbon flux during photosynthesis and improve our understanding of R L by explaining its metabolic origin, physiological significance in carbon balance during photosynthesis, and regulation under varying environmental conditions.