Nine-week-old, half-sib, yellow-poplar were exposed to O3 and/or SO2 (controls, 0.10 ppm O3, 0.08 ppm SO2 or 0.10 ppm O3 + 0.08 ppm SO2, 4 hr/d, 5d/wk) in combination with simulated rain (pH 3.0, 4.3 or 5.6, 1 hr/d, 2 d/wk at 0.75 cm/hr) for 6 wk, with rain applied either just before or after fumigation. Across all rain treatments, O3 + SO2 resulted in a significant decrease in height growth and dry matter production and an increase in leaf area ratio (LAR) compared with controls. The combined effect of O3 and SO2 was additive in nature, except for root dry weight and leaf weight ratio (LWR) in which a greater than additive response was observed. The alteration in LAR was due to dry matter remaining in the leaves as indicated by an increase in LWR. Rain treatments altered leaf dry weight which appeared as a quadratic function across all pollutant treatments. A significant gaseous pollutant × rain pH interaction occurred for root dry weight, leaf area increase, mean relative growth rate (RGR), mean unit leaf rate (ULR) and chlorophyll content. Ozone exposure resulted in a linear decrease in root dry weight, leaf area increase, RGR and ULR, as the acidity of rain increased (decreased pH), whereas fumigation with SO2 resulted in an increase in these growth variables as rain pH decreased. Chlorophyll content increased in both O3 and SO2 treatments as the acidity of rain increased. Fumigation of wet leaves caused a significant reduction in dry matter production, shoot elongation, leaf area, RGR and ULR as compared with dry leaves across all treatments. The results of this study demonstrate that the effects of multiple pollutant stresses are more deleterious than any single pollutant exposure for these half-sib seedling trees. These findings are discussed relative to possible implications on the effects of gaseous pollutants in combination with acidic precipitation on yellow-poplar growth and productivity.
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