The role of stomata in sensitivity of Betula papyrifera seedlings to SO2 at different humidities.

Stomata of paper birch (Betula papyrifera Marsh.) seedlings were more open at high humidity than at low humidity and responded rapidly to changes in vapor pressure deficit. SO2 at 0.2 or 0.8 μl l-1 caused partial stomatal closure. Seedlings fumigated with SO2 at 0.2 or 0.5 μl l-1 for 30 h or 0.2 μl l-1 for 75 h took up more SO2 at high than at low humidity. Differences in pollutant uptake could be explained by stomatal conductance with no need to invoke changes in mesophyll conductance. Betula seedlings were more sensitive to SO2 when fumigated at high humidity, as manifested in more leaf necrosis, increased leaf abscission, and greater growth inhibition compared to seedlings fumigated at low humidity. Amount of injury to leaves increased with rate of SO2 uptake, and inhibition of root growth increased with total SO2 uptake.