OZONE-INDUCED CHANGES IN CHLOROPHYLL FLUORESCENCE KINETICS AND CO2 ASSIMILATION IN VICIA-FABA

After subsymptomatic exposure to 150ppb of ozone for a single 3-h period, a significant reduction in photosynthetic activity coupled with a reduction in stomatal conductance and transpiration was rapidly induced in two cultivars of broad bean, Vicia faba cvs. Reina blanca and Gigante d'Ingegnoli. The two cultivars behaved quite differently in the post-fumigation stages. Reina blanca recovered quickly, its photosynthetic rate returning to pre-fumigation values within 48 h. In Gigante d'Ingegnoli the recovery process took much longer; 72 h after ozonization the net photosynthesis was only 59 % that of the unfumigated controls. The chlorophyll fluorescence kinetics also showed significant alterations following the treatment, reflecting a perturbation in the photochemical functioning of the thylakoids, and specifically disturbances of the water-splitting enzyme system of the PSII. Such effects proved to be reversible in both cultivars, but the response of Reina blanca was much quicker, so that 72 h after the treatment normal kinetics had been restored. At the end of this same period the fluorescence behaviour of Gigante d'Ingegnoli was still altered, complete recovery taking 1 week. The contributions of stomatal conductance and photochemical quantum conversion to the observed reductions in photosynthetic rates also differed between the two cultivars. Thus, the monitoring of chlorophyll fluorescence is a simple and informative tool for the investigation of the subsymptomatic (latent) phytotoxic effects of the air pollutant ozone.