Predicting leaf-level fluxes of O3 and NO2:: the relative roles of diffusion and biochemical processes

Pollutants like O-3 and NO2 enter leaves through the stomata and cause damage during reactions with components of biological cell membranes. The steady-state flux rates of these gases into the leaf are determined by a series of physical and biochemical resistances including stomatal aperture, reactions occurring within the cell wall and the ability of the leaf to remove the products of apoplastic reactions. In the present study, multiple regression models incorporating stomatal conductance, apoplastic and symplastic ascorbate concentrations, and nitrate reductase (NR) activities were generated to explain the observed variations in leaf-level flux rates of O-3 and NO2. These measurements were made on the plant Catharanthus roseus (Madagascar periwinkle). The best-fit model explaining NO2 flux included stomatal conductance, apoplastic ascorbate and NR activity. This model explained 89% of the variation in observed leaf fluxes and suggested physical resistances, reaction between NO2 and apoplastic ascorbate, and the removal rate of nitrate (generated by reactions of NO2 and water) from the apoplast all play controlling roles in NO2 flux to leaves. O-3 flux was best explained by stomatal conductance and symplastic ascorbate explaining 66% of the total variation in leaf flux. Both models demonstrate the importance of measuring processes other than stomatal conductance to explain steady-state leaf-level fluxes of pollutant gases.