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Gas exchange rates of 4-week-old faba bean plants were measured after exposure to ozone (f20 /(g m ') in an open top chamber for 8 h per day over a period of 2 weeks. The exehange rates were compated with those of control plants. Plants exposed between mid-May and the end of July 1987 showed a tnitior negative efVect on stomataf conductance, while there was no effect on photosynthesis and respiration. Plants exposed between the end of August and early Oetober showed a negative effect on both stomatal conductance and photosytithesis. In addition, the dark respiration rate was slightly increased. It is concluded that ozone can have a direct eflect on the stomata as well as on the photosynthetic system and that the stomata are more sensitive than the photosynthetic system.

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