1991. Physiological effects of loog term exposure to low cotteentrations of SO, and NH, on poplar leaves. -Physiol. Plant 82: 32^0.Shoots of poplar {Populus euramericarm L. cv. Flevo) were exposed to filtered air, SO,, NH^ or a mixture of SO, and NH,, for 7 weeks in fumigation chambers. After this exposure gas exchange measurements were earried out using a leaf chamber. As cottipared to leaves exposed to filtered air, leaves pretreated with 112 (ig m"^ SO, showed a small redaction in maximum CO, assimilation rate (P^aJ and stomatal cotiduetanee (gJ. They also showed a slightly higher quantum yield and dark respiration. It! addition, the fluorescence measurements indicated that the CaSvin cycle of tlie leaves pretreated with 112 ng m^^ SO, was more rapidly activated after transition from dark to light. An exposure to 64 (ig m"^ NH, had a positive effeet on P^^,,, stomatal conductance and NHj uptake of the leaves. TTiis positive effect was counteracted by an SO, concetttration of 45 [ig m"^. The exposure treatments appeared to have no effect on the relationship between tiet CO,-assimilation and g^. Also, no injury of the leaf cuticle or of epiderttial eells was observed. Resistance analysis showed that NH3 transfer into the leaf can be estimated from data on the boundary layer aod stomatal resistance for H2O transfer and NH,. eoneentration at the leaf surfaee, irrespective of whether the leaves are exposed for a short or long time to NH, or to a mixture of NH, and SO,. In contrast SO, 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 SO, and HjO molecules is proposed.