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

Area of Science:

• Plant Physiology
• Biochemistry
• Photosynthesis Research

Background:

• Photosynthesis involves CO₂ uptake, but concurrent CO₂ release complicates measurements.
• Photorespiration is a known CO₂ release pathway, but other metabolic sources exist.
• Respiration in the light (R_L) is an independent CO₂ emission source during photosynthesis.

Purpose of the Study:

• Investigate the biochemical basis of respiration in the light (R_L).
• Revisit classical photosynthesis models with new isotopic and metabolic flux data.
• Clarify the role of R_L in carbon balance and its regulation.

Main Methods:

• Utilized isotopic labeling techniques.
• Employed metabolic flux analysis (MFA).
• Revisited and adapted classical photosynthesis models.

Main Results:

• Identified the cytosolic glucose-6-phosphate (G6P) shunt via the oxidative pentose phosphate pathway (OPPP) as the primary source of R_L.
• Explained anomalies in Calvin-Benson-Bassham (CBB) cycle labeling.
• Observed stable R_L under non-stressed conditions across CO₂ and light variations.
• Noted a shift in R_L to a plastidial source under heat stress.

Conclusions:

• The G6P shunt is the predominant metabolic origin of R_L.
• R_L plays a significant role in the carbon balance during photosynthesis.
• R_L regulation varies with environmental conditions, particularly heat stress.