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 Fv  : Fm 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.