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 peri od, a significant reduction in photosynthetic activity coupled with a reduction in stomatal conductance and transpiration was rapidly induce d in two cultivars of broad bean, Vicia faba cvs. Reina blanca and Gig ante d'Ingegnoli. The two cultivars behaved quite differently in the p ost-fumigation stages. Reina blanca recovered quickly, its photosynthe tic rate returning to pre-fumigation values within 48 h. In Gigante d' Ingegnoli the recovery process took much longer; 72 h after ozonizatio n the net photosynthesis was only 59 % that of the unfumigated control s. The chlorophyll fluorescence kinetics also showed significant alter ations following the treatment, reflecting a perturbation in the photo chemical 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 w as much quicker, so that 72 h after the treatment normal kinetics had been restored. At the end of this same period the fluorescence behavio ur of Gigante d'Ingegnoli was still altered, complete recovery taking 1 week. The contributions of stomatal conductance and photochemical qu antum conversion to the observed reductions in photosynthetic rates al so differed between the two cultivars. Thus, the monitoring of chlorop hyll fluorescence is a simple and informative tool for the investigati on of the subsymptomatic (latent) phytotoxic effects of the air pollut ant ozone.