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Interspecific Variation in SO(2) Flux : Leaf Surface versus Internal Flux, and Components of Leaf Conductance.

The objective of this study was to clarify the relationships among stomatal, residual, and epidermal conductances in determining the flux of SO(2) air pollution to leaves. Variations in leaf SO(2) and H(2)O vapor fluxes were determined using four plant species: Pisum sativum L. (garden pea), Lycopersicon esculentum Mill. flacca (mutant of tomato), Geranium carolinianum L. (wild geranium), and Diplacus aurantiacus (Curtis) Jeps. (a native California shrub). Fluxes were measured using the mass-balance approach during exposure to 4.56 micromoles per cubic meter (0.11 microliters per liter) SO(2) for 2 hours in a controlled environmental chamber. Flux through adaxial and abaxial leaf surfaces with closed stomata ranged from 1.9 to 9.4 nanomoles per square meter per second for SO(2), and 0.3 to 1.3 millimoles per square meter per second for H(2)O vapor. Flux of SO(2) into leaves through stomata ranged from approximately 0 to 8.5 (dark) and 3.8 to 16.0 (light) millimoles per square meter per second. Flux of H(2)O vapor from leaves through stomata ranged from approximately 0 to 0.6 (dark) to 0.4 to 0.9 (light) millimole per square meter per second. Lycopersicon had internal flux rates for both SO(2) and H(2)O vapor over twice as high as for the other species. Stomatal conductance based on H(2)O vapor flux averaged from 0.07 to 0.13 mole per square meter per second among the four species. Internal conductance of SO(2) as calculated from SO(2) flux was from 0.04 mole per square meter per second lower to 0.06 mole per square meter per second higher than stomatal conductance. For Pisum, Geranium, and Diplacus stomatal conductance was the same or slightly higher than internal conductance, indicating that, in general, SO(2) flux could be predicted from stomatal conductance for H(2)O vapor. However, for the Lycopersicon mutant, internal leaf conductance was much higher than stomatal conductance, indicating that factors inside leaves can play a significant role in determining SO(2) flux.