In Vicia faba leaves photoinhibition from ozone fumigation in light precedes a decrease in quantum yield of functional PSII centres

Treatment with a single pulse of O-3 (150 ppb, 4 h) on Vicia faba (cv. Giga nte d'Ingegnoli) plants induced a decrease in photosynthetic rate at light saturation level but only slight changes in stomatal conductance and transp iration rate. The increase in intercellular CO2 concentration implied that the predominant factor responsible for the reduction in light-saturated CO2 uptake was a decrease in the carboxylation efficiency. A slight change in the photochemical efficiency of PS2 (F-v/F-m) that decreased in treated pla nts more than did the quantum yield of functional centres (Delta F/F'(m)) l asted hours after treatment termination; photosynthetic efficiency decrease occurred after an increase in F-o. Thermal dependence of fluorescence yiel d of open PS2 reaction cent res was modified by O-3. This eventually led to photosensitizing these leaves to photoinhibition but quantum yield of func tional centres was again less sensitive to this combination. Increase in el ectrolyte leakage kinetics in distilled water indicated that, in the stress ed plants, damage at the membrane level was occurring and this was confirme d also by the thiobarbituric reactive substance increase in ozonated leaves , taken as indicators of membrane lipid peroxidation.