Characterisation of the photosynthetic response of tobacco leaves to ozone: CO2 assimilation and chlorophyll fluorescence

The present paper reports on the characterisation of the photosynthetic responses to a single pulse of ozone (150 ppb for 5 hours) in the leaves of two tobacco cultivars displaying different degrees of sensitivity to O3 (BelB, O3-resistant and BelW3, O3-sensitive). In the BelW3 cultivars, O3 induced a decrease in the photosynthetic rate, but not in actual PSII efficiency at steady-state photosynthesis. The reduction state of QA did not change in these plants while a strong decrease in intrinsic PSII efficiency was observed. The quantum yield for photosynthetic CO2 assimilation decreased more than the actual PSII efficiency, suggesting the presence of a significant fraction of electron transport to molecular oxygen or the existence of some form of cyclic electron flow. O3-treated leaves reduced the excess of absorbed light (i.e. light that couldnʹt be used in photosynthesis) by dissipating a large part of the light absorbed by the PSII antenna as heath. In BelB, the CO2 assimilation rate did not change in the presence of the pollutant O3 and the only gas exchange parameter that changed was the stomatal conductance, which significantly increased. Some of the Chl fluorescence parameters changed after O3 fumigation, but returned to values similar to the controls when measured 24 hours after removing the stress. The only Chl fluorescence parameter that significantly increased during the recovery phase was the 1-qP