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

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Abstract
This study investigated ways in which genetically determined differences in SO2 susceptibility resulting from ecotypic differentiation inGeranium 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. When exposed to SO2, both ecotypes re-emit sulfur in a volatile, reduced form, presumably as H2S. Because H2S efflux rates at various SO2 concentrations were comparable between ecotypes, genetic differences inG. carolinianum could not be attributed to a re-emission of excess sulfur as H2S. Incipient SO2 effects on photosynthesis were observed as cumulative SO2 flux into the leaf interior excecded 0.40 nmol·m−2 in the resistant ecotype and 0.26 nmol·m−2 in the sensitive ecotype. Although initial SO2-induced changes in photosynthesis in both ecotypes were mediated through an increase in stomatal resistance to CO2, the ecotype-specific patterns as a function of pollutant concentration and exposure time were associated with marked increases in residual resistance to CO2. Patterns in photosynthesis, photoassimilate retention, and growth following long-term SO2 exposures were also ecotype-specific. 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 inG. 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.