Effects of heat stress on gas exchange and photosystem II (PSII) photochemical activity of Phillyrea angustifolia exposed to elevated CO2 and subsaturating 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 40 °C) on photosynthesis and photosystem II (PSII) photochemical efficiency of plants grown at ambient CO2 and exposed to an elevated 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 net 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 reductive power was more utilized in non-assimilatory processes than in CO2 fixation at both CO2 treatments. This result suggested that impairment of the Calvin cycle rather than electron transport inhibition was the main cause of the limitation in CO2 fixation.