Assimilation of CO₂ , enzyme activation and photosynthetic electron transport in bean leaves, as affected by high light and ozone

• Bean seedlings (Phaseolus vulgaris cv. Pinto) were grown in the greenhouse at a light intensity of 400 µmol m^-2 s^-1 . When the primary leaf was fully expanded, plants were divided into four groups and subjected to one of the following treatments: light intensity of 400 µmol m^-2 s^-1 and filtered air (control); light intensity of 400 µmol m^-2 s^-1 and ozone (O₃ ) (150 nl l^-1 for 5 h) (ozonated); light intensity of 1000 µmol m^-2 s^-1 for 5 h and filtered air (HL); and light intensity of 1000 µmol m^-2 s^-1 and O₃ (150 nl l^-1 ) for 5 h (HL + O₃ ). • At the end of the treatments (HL and/or O₃ ) a strong decrease in CO₂ assimilation rate as well a decrease in stomatal conductance were observed, while no changes in intercellular CO₂ concentration were recorded. In addition the F_v : F_m ratio (maximal quantum yield for PSII photochemistry) decreased in the stressed leaves (HL and/or O₃ ), indicating photoinhibition, and they showed a corresponding increase in minimal fluorescence (F₀ ), indicating a higher number of deactivating photosystem II (PSII) centres. • The maximum catalytic activity of the Benson-Calvin cycle enzymes, fructose-1,6-bisphosphate phosphatase (FBPase) and Rubisco, decreased following HL + O₃ stress but activation was enhanced. A linear relation was found between activation state of NADP-malate dehydrogenase (MDH) and the flux of electrons through PSII and in HL + O₃ -treated plants NADP-MDH activity decreased at high irradiance levels, indicating a limitation in linear electron flux.