Visible injury and growth responses of young cuttings of Populuscanadensis and P. nigra to nitrogen dioxide and sulphur dioxide

Carolina poplar (Populuscanadensis Moensch 'Eugenei') and Lombardy poplar (Populusnigra L. 'Italica') were exposed to NO2 and SO2, separately or in combination, for 1 h using concentrations of 0.50 and 1.0 μL•L−1 in separate experiments to determine if foliar injury symptoms can be used as a surrogate for growth effects and if single pollutants have noninteractive effects when mixed. Lombardy poplar was more sensitive than Carolina poplar to exposures to combined SO2 and NO2 at 1.0 μL•L−1 based on visible foliar injury symptoms. In contrast, leaf growth in both species was significantly stimulated by NO2 at 0.50 μL•L−1 and stem growth significantly decreased by NO2 at 1.0 μL•L−1. There was little effect of SO2; there were significant interactions of NO2 and SO2 only in Lombardy poplar exposed to 0.50 μL•L−1. These results suggest that growth effects can not be predicted by foliar injury symptoms and that few significant growth effects are identifiable in short-term studies.