Growth and physiological responses of yellow-poplar seedlings exposed to ozone and simulated acidic rain.

Nine-week-old, yellow-poplar (Liriodendron tulipifera L.) seedlings were exposed to ozone (O(3)) 4 h day(-1), 5 days week(-1), in combination with simulated rain (pH 3.0, 4.3, or 5.6, 1 h day(-1), 2 days week(-1), at 0.75 cm h(-1)) for 6 weeks, under controlled laboratory conditions. There was no main treatment effect of O(3) (<0.02, 0.05, 0.10, or 0.15 microlitre(-1)) on height growth or total plant biomass. However, specific leaf area increased linearly (P=0.05) with exposure of plants to increasing O(3) concentrations. Exposure of seedlings to 0.10 microl litre(-1) O(3) reduced the leaf area ratio and the stomatal conductance (S(c)), but increased the mean unit leaf rate compared to control plants. This O(3) concentration also caused S(c) to decrease more rapidly, relative to non-fumigated plants, in response to increasing vapor pressure deficit. The only main treatment effect caused by simulated rain was a linear decrease in S(c) with increasing acidity. For combined pollutant effects, O(3) applied at 0.05 or 0.10 microl litre(-1) resulted in significant (P=0.05) linear decreases in dry weights and relative growth rates (RGR) of stems and leaves, and in cumulative leaf area (LAI), as the solution pH decreased. Root to shoot ratio (RSR) decreased linearly, whereas stem RGR and LAI increased linearly in response to decreasing pH for seedlings exposed to 0.15 microl litre(-1) O(3). Seedlings treated with a pH 5.6 solution exhibited a linear decrease (P=0.05) in leaf dry weight and RGR and a linear increase (P=0.05) in RSR as O(3) concentrations increased.