Changes in yellow poplar stomatal resistance with SO2 and O3 fumigation

The effect of O₃ and SO₂ on leaf diffusive resistance (LDR) of yellow poplar seedlings was studied at 40% and 80% humidity. LDR was measured at 0800, 0900, 1100, 1400 and 1600 h on seedlings fumigated from 0900 h until 1400 h each day for five consecutive days. Fumigation treatments were control, 0·15 μl litre⁻¹ O₃, 0·25 μl litre⁻¹ SO₂ and 0·15 μl litre⁻¹ O₃ + 0·25 μl litre⁻¹ SO₂. No change in daily LDR response was observed for the seedlings at 40% humidity. At 80% humidity, daily LDR response of seedlings in all four treatments changed significantly over the 5 days of the experiment. The range of the LDR values and the daily response curves both changed. The higher humidity apparently caused the stomata to open so that more pollutants could enter the leaves. The pollutants then interacted with the leaf cells and modified the stomatal response.