Effects of heat stress on gas exchange and PSII photochemical activity of Phillyrea angustifolia exposed to elevated CO2 and sub-saturating irradiance

Gas exchange and chlorophyll a fluorescence measurements were performed simultaneously on leaves of Phillyrea angustifolia L. to assess the effects of heat stress (30 min at 45° C) on photosynthesis and photosystem II (PSII) photochemical efficiency on plants grown at ambient CO2 concentration (800 μmol mol-1) and 300 μmol photons m-2 s-1. No significant difference was found in the heat-induced decreases of photosynthesis (PN), quantum yield of PSII electron transport (ΦPSII), and maximum PSII photochemical efficiency (Fv/Fm) between plants exposed to ambient and elevated CO2 concentrations, showing that elevated CO2 was not able to reduce the potential for photoinhibition at high temperatures under moderate light conditions. The heat-induced decrease of PN was higher than that of ΦPSII indicating that the reductive power was more utilized in non-assimilatory processes than in CO2 fixation at both CO2 treatments. This result suggests that the impairment of the Calvin cycle rather than electron transport inhibition was the main cause of the limitation of the CO2 fixation.