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Effects of ozone on spring wheat (Triticum aestivum L. cv. Satu) were studied in an open-top chamber experiment during two growing seasons (1992)(1993) at Jokioinen in south-west Finland. The wheat was exposed to filtered air (CF), non-filtered air (NF), non-filtered airj35 nl l −" ozone for 8 h d −" (NF + ) and ambient air (AA). Each treatment was replicated five times. Two wk after anthesis, after 4 wk of ozone treatment (NF + , 45 nl l −" 1000-1800 hours, seasonal mean) the net CO # uptake of wheat flag leaves was decreased by c. 40 % relative to CF and NF treatments, both initial and total activity of Rubisco and the quantity of protein-bound SH groups were decreased significantly. Added ozone also significantly accelerated flag leaf senescence recorded as a decrease in chloroplast size. The effect was significant 2 wk after anthesis, and senescence was complete after 4 wk. In the CF and NF treatments senescence was complete 5 wk after anthesis. The significant effect of ozone on the chloroplasts and net CO # uptake 2 wk after anthesis did not affect the grain filling rate. However, since the grain filling period was shorter for ozone fumigated plants, kernels were smaller. The decrease in 1000-grain weight explained most of the yield reduction in the plants under NF + treatment. The results indicate that wheat plants are well buffered against substantial decrease in source activity, and that shortened flag leaf duration is the major factor causing ozone-induced yield loss.