The study investigated ways in which genetically determined differences in SO2 susceptibility resulting from ecotypic differentiation in Geranium carolinianum were expressed physiologically. The SO2-resistant and SO2-sensitive ecotypes were exposed to a combination of short- and long-term SO2 exposures to evaluate the responses of photosynthesis, H2S efflux from foliage (sulfur detoxification), photoassimilate retention, leaf-diffusive resistance to CO2, and growth. Although physiological accommodation of SO2 stress was observed in both ecotypes, it was more pronounced in the resistant ecotype. The physiological mechanisms underlying genetic differences in G. carolinianum in response to SO2 stress were concluded to be (1) dissimilar threshold levels of response to SO2 and/or its toxic derivatives and (2) differences in homeostatic processes governing the rate of repair or compensation for physiological injury.