Inhibition of photosynthesis ofPopulus euramericana andHelianthus annuus by SO2, NO2 and O3

Populus euramericana cv. I-214 andHelianthus annuus L. cv. Russian Mammoth were exposed to various concentrations of O₃ SO₂ or NO₂ for 2 h in a cylindrical assimilation chamber. The threshold concentrations of air pollutants for inhibition of net photosynthesis differed between the two species and also between the pollutants tested. Furthermore, the lethal concentrations where the net photosynthetic rates were completely inhibited, also differed between species and between pollutants. For SO₂ and NO₂,P. euramericana was more tolerant photosynthetically thanH. annuus when related to the concentration of pollutants used during the experiment. However, when related to the cumulative uptake rate of each pollutant, the photosynthetic tolerance of the two species was similar. In contrast to the effects of SO₂ or NO₂, the influence of O₃ on net photosynthesis was quite different. The relative rates of net photosynthesis in both species showed the same linear relationship with O₃ concentration. However, the relationship between the relative rate of net photosynthesis and the cumulative uptake rate of O₃ differed between the two species, although it was linear in both cases.