PHYSIOLOGICAL-EFFECTS OF LONG-TERM EXPOSURE TO LOW CONCENTRATIONS OF SO2 AND NH3 ON POPLAR LEAVES

Shoots of poplar (Populus euramericana L. cv. Flevo) were exposed to filtered air, SO2, NH3 or a mixture of SO2 and NH3 for 7 weeks in fumigation chambers. After this exposure gas exchange measurements were carried out using a leaf chamber. As compared to leaves exposed to filtered air, leaves pretreated with 112-mu-g m-3 SO2 showed a small reduction in maximum CO2 assimilation rate (P(max)) and stomatal conductance (g(s)). They also showed a slightly higher quantum yield and dark respiration. In addition, the fluorescence measurements indicated that the Calvin cycle of the leaves pretreated with 112-mu-g m-3 SO2 was more rapidly activated after transition from dark to light. An exposure to 64-mu-g m-3 NH3 had a positive effect on P(max), stomatal conductance and NH3 uptake of the leaves. This positive effect was counteracted by an SO2 concentration of 45-mu-g m-3. The exposure treatments appeared to have no effect on the relationship between net CO2-assimilation and g(s). Also, no injury of the leaf cuticle or of epidermal cells was observed.Resistance analysis showed that NH3 transfer into the leaf can be estimated from data on the boundary layer and stomatal resistance for H2O transfer and NH3 concentration at the leaf surface, irrespective of whether the leaves are exposed for a short or long time to NH3 or to a mixture of NH3 and SO2. In contrast SO2 uptake into the leaves was only partly correlated to the stomatal resistance. The results suggest a large additional uptake of this gas by the leaves. The possibility of a difference in path length between SO2 and H2O molecules is proposed.