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 O-3 (BelB, O-3-resistant and BelW3, O-3-sensitive). In the BelW3 cultivars, O-3 induced a decrease in the photosynthetic rate, but not in actual PSII efficiency at steady-state photosynthesis. The reduction state of Q(A) 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. O-3-treated leaves reduced the excess of absorbed light (i.e. light that couldn't 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 O-3 and the only gas exchange parameter that changed was the stomatal conductance, which significantly increased. Some of the Chl fluorescence parameters changed after O-3 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-q(P).