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CO2 vs. SO2: Effects on Photosynthesis Rates of Field-Grown Soybeans
Reference
Lee, E.H., Pausch, R.C., Rowland, R.A., Mulchi, C.L. and Rudorff, B.F.T. 1997. Responses of field-grown soybean (cv. Essex) to elevated SO2 under two atmospheric CO2 concentrations. Environmental and Experimental Botany 37: 85-93.

Background
The authors note that "several studies using controlled environment chambers have shown that CO2 may compensate for sulfur dioxide (SO2)-induced leaf injury (Black, 1982; Carlson and Bazzaz, 1982; Darrall, 1989; Sandhu et al., 1992; Rao and De Kok, 1994; Niewiadomska and Miszalski, 1995; Tausz et al., 1996)." Consequently, they decided to conduct a similar study out-of-doors using open-top chambers placed over soybeans (Glycine max L. Merr. cv. Essex) growing in a typical field situation to see if the positive effects of atmospheric CO2 enrichment would compensate for the negative effects of elevated SO2 concentrations on soybean photosynthetic rates.

What was done
Soybeans were grown for a full season in open-top chambers under normal field conditions while exposed to either ambient (350 ppm) or elevated (500 ppm) atmospheric CO2 concentrations in combination with atmospheric SO2 concentrations of either 0.00 or 0.12 ppm, which latter concentration was described by Lee et al. as being sufficient to produce "potentially toxic effects over long term exposure."

What was learned
Throughout the period of pod filling, the mean photosynthetic rate of plants growing in ambient-CO2 but elevated-SO2 air was 17.2% lower than the mean rate of plants growing in ambient-CO2 and SO2-free air, while the mean photosynthetic rate of plants growing in CO2-enriched but SO2-free air was 25.1% higher than the mean rate of plants growing in ambient-CO2 and SO2-free air. Most important of all, the mean photosynthetic rate of plants growing in CO2-enriched and elevated-SO2 air was 33.4% greater than the mean rate of plants growing in ambient-CO2 and SO2-free air. Hence, enriching the air with CO2 more than compensated for the negative effects of elevated SO2 on photosynthesis rates of soybeans in this study.

What it means
CO2 is clearly up to the task of ameliorating the negative effects of SO2 pollution on leaf photosynthetic rates of soybeans and then some, as it boosted rates even higher than they were in the total absence of the pollutant.

References
Black, V.J. 1982. Effects of sulphur dioxide on physiological processes in plants. In: Unsworth, M.H. and Ormrod, O.P. (Eds.), Effects of Gaseous Pollution in Agriculture and Horticulture, Butterworth, London, UK, pp. 67-91.

Carlson, R.W. and Bazzaz, F.A. 1982. Photosynthetic and growth responses to fumigation with SO2 at elevated CO2 for C3 and C4 plants. Oecologia 54: 50-54.

Darrall, N.M. 1989. The effect of air pollutants on physiological processes in plants. Plant, Cell and Environment 12: 1-30.

Niewiadomska, E. and Miszalski, Z. 1995. Does CO2 modify the effect of SO2 on variegated leaves of Chlorophytum comosum (Thunb) Bak? New Phytologist 130: 461-466.

Rao, M.V. and De Kok, L.J. 1994. Interactive effects of high CO2 and SO2 on growth and antioxidant levels in wheat. Phyton (Horn) 34: 279-290.

Sandhu, R., Li, Y. and Gupta, G. 1992. Sulphur dioxide and carbon dioxide induced changes in soybean physiology. Plant Science 83: 31-34.

Tausz, M., De Kok, L., Stulen, I. and Grill, D. 1996. Physiological responses of Norway spruce trees to elevated CO2 and SO2. Journal of Plant Physiology 148: 362-376.

Reviewed 10 January 2007