Stomatal Conductance and Sulfur Uptake of Five Clones of Populus tremuloides Exposed to Sulfur Dioxide.

Plants of five clones of Populus tremuloides Michx. were exposed to 0, 0.2 or 0.5 microliter per liter SO(2) for 8 hours in controlled environment chambers. In the absence of the pollutant, two pollution-resistant clones maintained consistently lower daytime diffusive conductance (LDC) than did a highly susceptible clone or two moderately resistant clones. Differences in LDC among the latter three clones were not significant. At 0.2 microliter per liter SO(2), LDC decreased in the susceptible clone after 8 hours fumigation while the LDC of the other clones was not affected. Fumigation with 0.5 microliter per liter SO(2) decreased LDC of all five clones during the fumigation. Rates of recovery following fumigation varied with the clone, but the LDC of all clones had returned to control values by the beginning of the night following fumigation. Night LDC was higher in the susceptible clone than in the other clones. Fumigation for 16 hours (14 hours day + 2 hours night) with 0.4 microliter per liter SO(2) decreased night LDC by half. Sulfur uptake studies generally confirmed the results of the conductance measurements. The results show that stomatal conductance is important in determining relative susceptibility of the clones to pollution stress.