Abstract
AbstractDuring oxygenic photosynthesis, oxygen (O2) is generated from water photolysis, which provides reducing power to sustain CO2assimilation. To date, traditional leaf gas-exchange experiments have been focused on net CO2exchange (Anet), with limited observations of net oxygen production (NOP). Here, we present the first gas-exchange/fluorescence system, coupling CO2/H2O analysis (photosynthesis and transpiration) with NOP and isoprene emission measurements. This configuration allowed us to calculate the assimilatory quotient (AQ = Anet/NOP) and thus obtain a more complete picture of the photosynthetic redox budget via photosynthetic production of O2, electron transport rate (ETR), and isoprene biosynthesis. We used cottonwood leaves (Populus trichocarpa) and carried out response curves to light, CO2and temperature along with18O-labelling with18O-enriched water. We found that Anetand NOP were linearly correlated across environmental variables with AQ of 1.27 +/- 0.12 regardless of light, CO2, and temperature. Anetand NOP had optimal temperatures (Topt) of 31°C, while ETR (35°C) and isoprene emissions (39°C) had distinctly higher Topt. Leaves labelled with H218O produced labeled (18O16O) oxygen with the same Toptas ETR (35°C). The results confirm a tight connection between water oxidation and ETR and are consistent with a suppression of Anetand 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 CO2and O2recycling.KeywordsPhotosynthesis, net oxygen production, gross oxygen production, H218O labelingOne sentence summaryA leaf gas-exchange system is presented enabling a more complete picture of the photosynthetic redox budget and calculation of the assimilatory quotient.
Publisher
Cold Spring Harbor Laboratory