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 R_d) or respiration in the light (R_L)-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 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.