Photosynthesis, carbon allocation, and growth of sulfur dioxide ecotypes ofGeranium carolinianum L.

This study investigated ways in which genetically determined differences in SO susceptibility resulting from ecotypic differentiation inGeranium carolinianum were expressed physiologically. The SO-resistant and SO-sensitive ecotypes were exposed to a combination of short- and long-term SO exposures to evaluate the responses of photosynthesis, HS efflux from foliage (sulfur detoxification), photoassimilate retention, leaf-diffusive resistance to CO, and growth. When exposed to SO, both ecotypes re-emit sulfur in a volatile, reduced form, presumably as HS. Because HS efflux rates at various SO concentrations were comparable between ecotypes, genetic differences inG. carolinianum could not be attributed to a re-emission of excess sulfur as HS. Incipient SO effects on photosynthesis were observed as cumulative SO flux into the leaf interior excecded 0.40 nmol·m in the resistant ecotype and 0.26 nmol·m in the sensitive ecotype. Although initial SO-induced changes in photosynthesis in both ecotypes were mediated through an increase in stomatal resistance to CO, the ecotype-specific patterns as a function of pollutant concentration and exposure time were associated with marked increases in residual resistance to CO. Patterns in photosynthesis, photoassimilate retention, and growth following long-term SO exposures were also ecotype-specific. Although physiological accommodation of SO stress was observed in both ecotypes, it was more pronounced in the resistant ecotype. The physiological mechanisms underlying genetic differences inG. carolinianum in response to SO stress were concluded to be (1) dissimilar threshold levels of response to SO and/or its toxic derivatives and (2) differences in homeostatic processes governing the rate of repair or compensation for physiological injury.