Stomatal response of three birch species exposed to varying acute doses of SO₂

Seedlings of 3 birch species were exposed to either 0.3, 0.6, 0.9, or 1.2 ppm SO₂ for 1, 2, 3, or 4 hours. Stomatal conductance rate measurements of 10 plants were taken prior to and immediately following each exposure. The percentage of leaf tissue injured by SO₂ was estimated 72 hours after exposure. Stomatal conductance rates of European white birch (Betula pendula Roth.) and yellow birch (B. lutea Michx. f.) increased after exposure to 0.3 ppm SO₂ for 1 and 2 hours, and decreased in response to all other doses of SO₂. Stomatal conductance rates of gray birch (B. populifolia Marsh.) increased only after exposure to 0.6 ppm SO₂ for 1 and 3 hours and decreased in response to all other dosages. European white birch was slightly more susceptible to SO₂ than gray birch, whereas yellow birch was tolerant.