Effects of low‐level ozone exposure under ambient conditions on photosynthesis and stomatal control of Vicia faba L.

Gas exchange rates of 4‐week‐old faba bean plants were measured after exposure to ozonc (120 μg m3) in an open top chamber for 8 h per day over a period of 2 weeks. The exchange rates were compared with those of control plants. Plants exposed between mid‐May and the end of July 1987 showed a minor negative effect on stomatal conductance, while there was no effect on photosynthesis and respiration. Plants exposed between the end of August and early October showed a negative effect on both stomatal conductance and photosynthesis. In addition, the dark respiration rate was slightly increased. It is concluded that ozone can have a direct effect on the stomata as well as on the photosynthetic system and that the stomata are more sensitive than the photosynthetic system.