Effects of sulfur dioxide and ozone on Ulmus americana seedlings. I. Visible injury and growth

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Abstract
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-month-old, actively growing and quiescent Ulmus americana L. seedlings. In actively growing seedlings visible injury to leaves from SO2–O3 mixtures was evident within 24 h; from O3 within 36 to 48 h; and from SO2 within 48 h after fumigation ceased. Sulfur dioxide and SO2–O3 treatments reduced expansion of new leaves (< 1 cm long at time of fumigation) within 1 week, but normal rates of leaf expansion were restored by the end of the 2nd week. 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 week. After 5 weeks, 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. However, all fumigation treatments induced severe defoliation in quiescent plants.
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