Does elevated CO2 protect photosynthesis from damage by high temperature via modifying leaf water status in maize seedlings?

We hypothesized that decreased stomatal conductance (g s) at elevated CO2 might decrease transpiration (E), increase leaf water potential (ΨW), and thereby protect net photosynthesis rate (P N) from heat damage in maize (Zea mays L) seedlings. To separate long-term effects of elevated CO2, plants grew at either ambient CO2 or elevated CO2. During high-temperature treatment (HT) at 45°C for 15 min, leaves were exposed either to ambient CO2 (380 μmol mol-1) or to elevated CO2 (560 μmol mol-1). HT reduced P N by 25 to 38% across four CO2 combinations. However, the g s and E did not differ among all CO2 treatments during HT. After returning the leaf temperature to 35°C within 30 min, g s and E were the same or higher than the initial values. Leaf water potential (ΨW) was slightly lower at ambient CO2, but not at elevated CO2. This study highlighted that elevated CO2 failed in protecting P N from 45°C via decreasing g s and ΨW.