Concurrent measurement of O2and isoprene production during photosynthesis: pros, cons, and metabolic implications

During oxygenic photosynthesis, oxygen (O 2 ) is generated from water photolysis, which provides reducing power to sustain CO 2 assimilation. To date, traditional leaf gas-exchange experiments have been focused on net CO 2 exchange (A net ), with limited observations of net oxygen production (NOP). Here, we present the first gas-exchange/fluorescence system, coupling CO 2 /H 2 O analysis (photosynthesis and transpiration) with NOP and isoprene emission measurements. This configuration allowed us to calculate the assimilatory quotient (AQ = A net /NOP) and thus obtain a more complete picture of the photosynthetic redox budget via photosynthetic production of O 2 , electron transport rate (ETR), and isoprene biosynthesis. We used cottonwood leaves ( Populus trichocarpa ) and carried out response curves to light, CO 2 and temperature along with 18 O-labelling with 18 O-enriched water. We found that A net and NOP were linearly correlated across environmental variables with AQ of 1.27 +/- 0.12 regardless of light, CO 2 , and temperature. A net and NOP had optimal temperatures (T opt ) of 31°C, while ETR (35°C) and isoprene emissions (39°C) had distinctly higher T opt . Leaves labelled with H 2 18 O produced labeled ( 18 O 16 O) oxygen with the same T opt as ETR (35°C). The results confirm a tight connection between water oxidation and ETR and are consistent with a suppression of A net and NOP at high temperature driven by an acceleration of (photo)respiration. The findings support the view of isoprene biosynthesis primarily driven by excess photosynthetic ATP/NADPH not consumed by the Calvin cycle during photorespiratory conditions as an important thermotolerance mechanism linked with high rates of CO 2 and O 2 recycling. Keywords Photosynthesis, net oxygen production, gross oxygen production, H 2 18 O labeling One sentence summary A leaf gas-exchange system is presented enabling a more complete picture of the photosynthetic redox budget and calculation of the assimilatory quotient.