Photosynthetic Temperature Tolerance Threshold Determines How Isoprene Emission is Affected by Elevated CO Concentration at High Temperatures.

The suppression of isoprene emissions by high CO levels can be mitigated by increasing temperature; however, little is known about why and to what extent species differ in their temperature-dependent release from high CO inhibition. We studied leaf photosynthetic characteristics and isoprene emissions over a 25°C-40°C temperature range at CO concentrations of 150, 400, and 1000 μmol mol in two species with contrasting heat resistance. In the temperate species , rising temperatures above 30°C shifted electron flow from photosynthesis to isoprene synthesis, reducing CO inhibition due to enhanced isoprene synthase activity and decreased sensitivity of the DMADP pool. Conversely, the tropical species showed greater heat tolerance in its photosynthetic apparatus, maintaining electron flow for CO fixation, and exhibited a consistent CO suppression of isoprene emissions throughout the experiment. These findings indicate that species differences in relative sensitivity of light and dark reactions of photosynthesis play crucial roles in modulating isoprene emissions under combined high CO and temperature conditions.