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Softwood cuttings of hybrid poplar clone No. 207 (Populusdeltoides Bartr. × Populustrichocarpa Torry and Gray) were fumigated with either charcoal-filtered air (control), 0.5 ppm SO2, or 0.5 ppm SO2 + 0.25 ppm O3 for 12 h each day for 3 weeks. The net photosynthetic rate and CO2 compensation point were then measured in a closed-loop gas assimilation system with an infrared gas analyzer. Net photosynthesis was measured at light intensities of 430 and 730 μE•m−2•s−1 (photosynthetically active radiation) and in CO2 concentrations of 300, 500, and 1000 ppm. Net photosynthesis increased with an increase in light intensity and CO2 concentration but was significantly reduced by the SO2 + O3 treatment. The leaves were classified into five groups on the basis of visible injury. There were no significant differences in the net photosynthetic rates among uninjured leaves in the three fumigation treatments. However, analysis of the data from the SO2 + O3 treatment, which separated the leaves into injury classes, showed that the CO2 – injury class interaction was significant. At 300 ppm CO2, there were no significant differences in the net photosynthetic rates among the five injury classes. At 500 and 1000 ppm CO2, there were significant differences in the photosynthetic rates between leaves without visible injury and those with injury. Photosynthesis in the injured leaves may have been suppressed by an increase in the respiration rate and a decrease in the photosynthetic area. The increased respiration rate is suggested by the CO2 compensation point data that was significantly higher in all of the fumigated leaves. There were no significant differences in the chlorophyll content of the leaves from the three treatment groups.

Hybrid poplar plants were exposed to 0.5 ppm SO2, 0.25 ppm O3 or 0.5 ppm SO2 + 0.25 ppm O3, 12 hr/day for 24 days to ascertain their effects on leaf growth and abscission. The data revealed that both O3 alone and O3 + SO2 promoted leaf abscission, while SO2 alone had no effect. Leaf area and dry weight were reduced while leaf abscission was stimulated by ozone fumigation. The interaction found between SO2 and O3, with all the parameters measured, was an antagonistic relationship in which SO2 reduced the toxic effect of ozone. The data analyzed, in relation to leaf position, demonstrated that ozone did not affect development of the six youngest rapidly growing leaves. However, ozone significantly reduced both leaf area and leaf weight, at later stages of development.