The effect of ozone in ambient air on ribulosebisphosphate carboxylase/oxygenase activity decreases photosynthesis and grain yield in wheat

Summary Wheat was grown in open-top chambers in the field and fumigated with charcoal-filtered air (0.02 µll -1 O 3 ), unfiltered air (0.035 µ ll -1 O 3 ) and air enriched with ozone (0.1 µll -1 O 3 [seasonal mean ozone concentration during 8 h per day, from 9 am to 5 pm]). Ambient plots were used for comparison. Dry matter production, grain yield, and the following photosynthetic parameters of flag leaves were determined: 14 CO 2 uptake in situ , CO 2 response of net photosynthesis in the laboratory, CO 2 compensation concentration, activity of ribulose hisphosphate carboxylase/oxygenase (RuBPCO) as initial activity after rapid extraction and as activity after incubation of the extract in the presence of CO 2 and Mg ++ , content of soluble protein and of chlorophyll, and stomatal resistance. Photosynthesis ( 14 CO 2 -fixation) of flag leaves at the beginning of anthesis and grain yield were highest in filtered air. In unfiltered air and in air enriched with ozone, photosynthesis was lower by 10% and 52%, respectively, and grain yield was lower by 12% and 57%, respectively. Net photosynthesis showed a proportional decrease at limiting and saturating C02 partial pressures. The decrease in net photosynthesis was associated with a decrease in the content of activatable RuBPCO and with an increase in the CO 2 compensation concentration. The degree of activation of RuBPCO in vivo increased with increasing ozone concentration. The content of soluble protein and of chlorophyll were similar in filtered and unfiltered air, slightly decreased in the open field plots and greatly decreased in ozone-enriched air. Stomatal resistance was only increased in ozone-enriched air. The results suggested that the response of photosynthesis to elevated ozone was the result of a limitation in the amount of RuBPCO present in the leaves and not of changes in CO 2 conductance or RuBP synthesis. Photosynthetic characteristics of leaves exposed to elevated ozone were similar to those found in senescing leaves. It is suggested that ozone induces premature senescence.