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