The effects of SO2 (2 ppm for 6 h), O3 (0.9 ppm for 5 h) and SO2-O3 mixtures (2 ppm SO2 and 0.9 ppm O3 for 5 h followed by SO2 for 1 h) were studied on injury and growth of 4 mo. old, actively growing and quiescent U. americana L. seedlings. In actively growing seedlings visible injury to leaves from SO2-O3 mixtures was evident within 24 h; from O3 within 36-48 h; and from SO2 within 48 h after fumigation ceased. SO2 and SO2-O3 treatments reduced expansion of new leaves (< 1 cm long at time of fumigation) within 1 wk, but normal rates of leaf expansion were restored by the end of the 2nd wk. Ozone had no significant effect on expansion of new leaves. Expansion of young leaves (> 1 cm long at time of fumigation) was markedly inhibited by all treatments; it was inhibited most by SO2-O3, an intermediate amount by SO2, and least by O3. Numbers of emerging leaves were significantly reduced by SO2 and by SO2-O3, but only at the end of the 1st wk. After 5 wk, stem dry weights were reduced by O3 and SO2-O3, and root dry weights were reduced by SO2 and SO2-O3. In quiescent plants pollution injury to leaves was similar to that in actively growing plants. All fumigation treatments induced severe defoliation in quiescent plants.
