Influence of irradiance on photosynthesis and PSII photochemical efficiency in maize during short-term exposure at high CO2 concentration

This work aimed to evaluate if gas exchange and PSII photochemical activity in maize are affected by different irradiance levels during short-term exposure to elevated CO 2 . For this purpose gas exchange and chlorophyll a fluorescence were measured on maize plants grown at ambient CO 2 concentration (control CO 2 ) and exposed for 4 h to short-term treatments at 800 μmol(CO 2 ) mol −1 (high CO 2 ) at a photosynthetic photon flux density (PPFD) of either 1,000 μmol m −2 s −1 (control light) or 1,900 μmol m −2 s −1 (high light). At control light, high-CO 2 leaves showed a significant decrease of net photosynthetic rate ( P N ) and a rise in the ratio of intercellular to ambient CO 2 concentration ( C i / C a ) and water-use efficiency (WUE) compared to control CO 2 leaves. No difference between CO 2 concentrations for PSII effective photochemistry (Φ PSII ), photochemical quenching (q p ) and nonphotochemical quenching (NPQ) was detected. Under high light, high-CO 2 leaves did not differ in P N , C i / C a , Φ PSII and NPQ, but showed an increase of WUE. These results suggest that at control light photosynthetic apparatus is negatively affected by high CO 2 concentration in terms of carbon gain by limitations in photosynthetic dark reaction rather than in photochemistry. At high light, the elevated CO 2 concentration did not promote an increase of photosynthesis and photochemistry but only an improvement of water balance due to increased WUE.