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 differentirradiance levels during short-term exposure to elevated CO2. For this purpose gas exchange and chlorophyll afluorescence were measured on maize plants grown at ambient CO2 concentration (control CO2) and exposed for 4 h toshort-term treatments at 800 ?mol(CO2) mol-1 (high CO2) at a photosynthetic photon flux density (PPFD) of either 1,000?mol m-2 s-1 (control light) or 1,900 ?mol m-2 s-1 (high light). At control light, high-CO2 leaves showed a significantdecrease of net photosynthetic rate (PN) and a rise in the ratio of intercellular to ambient CO2 concentration (Ci/Ca) andwater-use efficiency (WUE) compared to control CO2 leaves. No difference between CO2 concentrations for PSIIeffective photochemistry (?PSII), photochemical quenching (qp) and nonphotochemical quenching (NPQ) was detected.Under high light, high-CO2 leaves did not differ in PN, Ci/Ca, ?PSII and NPQ, but showed an increase of WUE. Theseresults suggest that at control light photosynthetic apparatus is negatively affected by high CO2 concentration in terms ofcarbon gain by limitations in photosynthetic dark reaction rather than in photochemistry. At high light, the elevated CO2concentration did not promote an increase of photosynthesis and photochemistry but only an improvement of waterbalance due to increased WUE.