Concurrent Measurement of O₂ Production and Isoprene Emission During Photosynthesis: Pros, Cons and Metabolic Implications of Responses to Light, CO₂ and Temperature

Traditional leaf gas exchange experiments have focused on net CO₂ exchange (A_net). Here, using California poplar (Populus trichocarpa), we coupled measurements of net oxygen production (NOP), isoprene emissions and δ¹⁸O in O₂ to traditional CO₂/H₂O gas exchange with chlorophyll fluorescence, and measured light, CO₂ and temperature response curves. This allowed us to obtain a comprehensive picture of the photosynthetic redox budget including electron transport rate (ETR) and estimates of the mean assimilatory quotient (AQ = A_net/NOP). We found that A_net and NOP were linearly correlated across environmental gradients with similar observed AQ values during light (1.25 ± 0.05) and CO₂ responses (1.23 ± 0.07). In contrast, AQ was suppressed during leaf temperature responses in the light (0.87 ± 0.28), potentially due to the acceleration of alternative ETR sinks like lipid synthesis. A_net and NOP had an optimum temperature (T_opt) of 31°C, while ETR and δ¹⁸O in O₂ (35°C) and isoprene emissions (39°C) had distinctly higher T_opt. The results confirm a tight connection between water oxidation and ETR and support a view of light-dependent lipid synthesis primarily driven by photosynthetic ATP/NADPH not consumed by the Calvin–Benson cycle, as an important thermotolerance mechanism linked with high rates of (photo)respiration and CO₂/O₂ recycling.