Effects of Low-Level SO2on Usnea hirta and Parmelia chlorochroa

Respiration rates of Usnea hirta (L.) Wigg. and Parmelia chlorochroa Tuck. exposed in the field to SO2fumigation decreased significantly, compared to control rates, within 30 days at 94 ppb SO2and within 60 days at 33 ppb. Nearly 100% of the algal cells were plasmolyzed within 60 days. The respiration rates of P. chlorochroa samples were not significantly different from those of control samples after 60 days of exposure to 18 ppb SO2. The respiration rates of U. hirta samples were significantly less than those of control samples after 96 days of exposure to 18 ppb SO2. A significantly greater percentage of the algal cells were plasmolyzed in U. hirta and P. chlorochroa, compared to control samples, after 31 days of exposure to 18 ppb. Damage to the lichens was more obvious and occurred faster than damage to adjacent vascular vegetation.