Effects of Sulfur Dioxide Fumigation on Photosynthesis, Respiration, and Chlorophyll Content of Selected Lichens

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Abstract
Four lichens [Parmelia bollianas Mull. Arg., Physcia stellaris (L.) Nyl., Xanthoria fallax (Hepp) Arn. and Physconia grisea (Lam.) Poelt] listed in order of decreasing mesophytism, were fumigated for 4 h at 0.01, 0.1, 1.0 and 2.5 ppm SO2 to determine the effects on photosynthesis, respiration and chlorophyll content. The lichens had been collected from Populus deltoides Marsh or Tilia americanas L. in northeastern South Dakota [USA] near a lignite-fired power generator. Photosynthesis decreased after fumigation at 1.0 and 2.5 ppm, but significant decreases occurred only after fumigation at 2.5 ppm. Expressed on the basis of per unit weight chlorophyll content, photosynthesis of Physconia grisea was most sensitive followed by that of Xanthoria fallax, Physcia stellaris and Parmelia bolliana. Expressed on the basis of per unit dry weight of lichen sample, photosynthesis of Physconia grisea was most sensitive followed by Xanthoria fallax, Physcia stellaris and Parmelia bolliana. In both cases, the more xerophytic species were more sensitive. Chlorophyll content in these species was not measurably altered by fumigation. Comparison of chlorophyll a and b absorption spectra peaks for fumigated and control samples indicated that no phaeophytinisation occurred. Insignificant and inconsistent differences in chlorophyll a/b ratios were observed. Respiration of Physcia stellaris and Parmelia bolliana decreased significiantly following fumigation with 2.5 ppm SO2; both species were more sensitive than Xanthoria fallax Physconia grisea was not tested for respiratory responses. The effects of SO2 fumigation on measured metabolic rates differed with the species. Photosynthetic rates of the xerophytic Xanthoria fallax and Physconia grisea were more sensitive than the more mesophytic Parmelias bolliana and Physcia stellaris. In contrast, respiratory sensitivities to SO2 fumigation were greater for P. bolliana and P. stellaris.