Photosynthesis in ozone‐exposed duckweed (Lemna gibba)

The photosynthetic light saturation curve in duckweed was lowered by 20–25% after ozone exposure (300 nmol mol −1 , 1 h). The light flux and oxygen concentration during ozone‐exposure had no effect on reduction of net photosynthesis. Net photosynthesis and photorespiration were both depressed by about 40% after exposure for 1 h to 360 nmol mol −1 ozone. We could not find any change in dark respiration after ozone exposure below 300 nmol mol −1 . When the concentration of ozone was doubled from 150 nmol mol −1 to 300 nmol mol −1 , the uptake of ozone in duckweed changed from 100 nmol m −2 s −1 to 170 nmol m −2 s −1 . We found no differences in fluorescence (pattern) between ozone treated plants and the control plants during a period of 150 min after ozone treatment, but there was an increase in synthesis of the Dl‐protein and a significant reduction in degradation after ozone treatment (300 nmol mol −1 , 1 h). These results, together with fluorescence measurements, indicate that photochemical electron transport was not responsible for the ozone‐induced reduction in net photosynthesis.