Two years later, the same scientists - Agrawal and Deepak (2003) - conducted a similar study of two cultivars of wheat (Triticum aestivum L. cv. Malviya 234 and HP1209) in open-top chambers maintained at atmospheric CO2 concentrations of 350 and 600 ppm both alone and in combination with 60 ppb SO2, in order to study the individual and interactive effects of elevated CO2 and SO2 on another of the world's major crops. This study revealed that the same exposure to elevated CO2 boosted photosynthetic rates by 58 and 48% in M234 and HP1209, respectively, while fumigation with elevated SO2 had no significant impact on rates of photosynthesis in either cultivar. However, plants grown in the combined treatment of elevated CO2 and elevated SO2 displayed photosynthetic rates that were 42 and 38% greater than those measured in control plants for M234 and HP1209, respectively.

Plants grown in elevated CO2 in this experiment also displayed an approximate 20% reduction in stomatal conductance, while those grown in elevated SO2 exhibited an average conductance increase of 15%. When exposed simultaneously to both gases, however, the plants displayed an average 11% reduction in stomatal conductance. And as a result, this phenomenon contributed to an approximate 32% increase in water-use efficiency (plant growth per unit of water used) for plants that were simultaneously exposed to increased concentrations of both gases, whereas plants exposed to elevated SO2 alone displayed an average decrease in water-use efficiency of 16%. In addition, plant exposure to elevated SO2 caused an average 13% decrease in foliar protein concentrations in both cultivars. However, when the same plants were concurrently exposed to an atmospheric CO2 concentration of 600 ppm, leaf protein levels only decreased by 3% in HP1209, while they actually increased by 4% in M234.

As the air's CO2 content continues to rise, therefore, it will likely allow these specific wheat cultivars and others to experience less stress and growth reductions as a consequence of SO2 pollution. In fact, Agrawal and Deepak's study demonstrates that CO2-induced increases in photosynthesis will only be partially offset by elevated SO2 concentrations, which should allow greater wheat yields to be produced in the future under similar conditions. In addition, since SO2-induced reductions in plant water-use efficiency were essentially eliminated by concurrent plant exposure to elevated CO2, these cultivars should be able to grow better in areas with limited water availability, as well as in areas close to industrial complexes emitting large quantities of SO2. Also, wheat plants growing in SO2-polluted air should not suffer as large a reduction in foliar protein content in a future high-CO2 world as they do currently.

Sandwiched between the two prior studies was the study of Izrael et al. (2002), who evaluated the health effects of sulfur dioxide pollution on Russian forests. Among other things, the five researchers noted that "sulfur dioxide (SO2) causes widespread damage to plants, because it can spread through large distances, and its emissions into the atmosphere are large." In 1996, for example, they found that "total SO2 emission from the industrial areas of Russia comprised 5866.76 thousand tons, or 42.2% of the total emission of liquid and gaseous pollutants." And in quantifying the extent and severity of this phenomenon, they determined that 1.3 million hectares of Russian forest land had been adversely affected by SO2 pollution. Additionally, they estimated that total forest destruction occurs on 2-5% of the above area, and that heavy, moderate and slight damage occur on 10-15%, 30-40% and 40-50% of this area, respectively.

These results indicate the seriousness of SO2 pollution for forest health; and they highlight the fact that atmospheric CO2 enrichment can significantly alleviate SO2's adverse biological consequences. Hallgren (1984), for example, demonstrated that a 300-ppm increase in the air's CO2 concentration stimulated the photosynthetic rate of Scots pine trees by 64% in unpolluted air and by 77% in air with abnormally high SO2 concentrations, while a 600-ppm increase in atmospheric CO2 stimulated photosynthetic rates in this important forest species by 85% in unpolluted air and by 110% in air of high SO2 concentration. Similarly, but with respect to agricultural species, Carlson (1983) found that a 900-ppm increase in the air's CO2 concentration boosted photosynthetic rates of soybeans by 87% in unpolluted air, but by a whopping 715% in high-SO2 air. Hence, the ongoing rise in the air's CO2 content can do much to either totally prevent or significantly alleviate the adverse consequences of anthropogenic SO2 pollution.

Last of all, we focus on studies conducted at naturally-occurring CO2 springs in Tuscany, Italy, that provide a unique opportunity to study the effects of long-term atmospheric CO2 enrichment on plant growth and development. However, these springs also emit elevated concentrations of the major phytotoxic air pollutants H2S and SO2 (Schulte et al., 1999); and, consequently, the springs also provide a perfect setting in which to study the relative strengths of two competing phenomena, i.e., the growth-promoting effect of elevated CO2 and the growth-retarding effects of elevated H2S and SO2.

Capitalizing on this situation, Grill et al. (2004) analyzed various properties of leaves and acorns produced on two species of oak tree (Quercus ilex L. and Quercus pubescens L.) growing in air of double-to-triple the normal atmospheric CO2 concentration near the CO2 springs, as well as the same characteristics of leaves and acorns growing on similar trees located some distance away in ambient-CO2 air. In addition, they analyzed several characteristics of seedlings they sprouted from acorns produced by the CO2-enriched and ambient-treatment trees; and they used chromosome stress tests "to investigate whether alterations in sulfur-regime have negative consequences for seedlings."

And what did they find? In the words of the six scientists, "acorns from CO2 springs contained significantly higher sulphur concentrations than controls (0.67 vs. 0.47 mg g^-1 dry weight in Q. ilex cotyledons and 1.10 vs. 0.80 in Q. pubescens)," indicative of the fact that the trees were indeed significantly affected by the H2S- and SO2-enriched air in the vicinity of the CO2-emitting springs. They also report that Q. ilix seedlings grown from CO2-spring acorns showed elevated rates of chromosomal aberrations in root tips, suggestive of a permanent stress. Nevertheless, as demonstrated by the results of several studies conducted on mature trees from these sites, the CO2-enriched air - even in the presence of significantly elevated concentrations of phytotoxic H2S and SO2 - tremendously enhanced the trees' photosynthetic prowess: by 26-69% (Blaschke et al., 2001), 36-77% (Stylinski et al., 2000), and a whopping 175-510% (Tognetti et al., 1998).

And thus it should be clear to all, especially to those who set policy at the U.S. Environmental Protection Agency or EPA, that not only is carbon dioxide not a "pollutant," as they claim it is, but that it is actually an anti-pollutant that helps plants to overcome the negative effects of real air pollutants.

References
Agrawal, M. and Deepak, S.S. 2003. Physiological and biochemical responses of two cultivars of wheat to elevated levels of CO2 and SO2, singly and in combination. Environmental Pollution 121: 189-197.

Blaschke, L., Schulte, M., Raschi, A., Slee, N., Rennenberg, H. and Polle, A. 2001. Photosynthesis, soluble and structural carbon compounds in two Mediterranean oak species (Quercus pubescens and Q. ilex) after lifetime growth at naturally elevated CO2 concentrations. Plant Biology 3: 288-297.

Carlson, R.W. 1983. The effect of SO2 on photosynthesis and leaf resistance at varying concentrations of CO2. Environmental Pollution Series A 30: 309-321.

Deepak, S.S. and Agrawal, M. 2001. Influence of elevated CO2 on the sensitivity of two soybean cultivars to sulphur dioxide. Environmental and Experimental Botany 46: 81-91.

Grill, D., Muller, M., Tausz, M. Strnad, B., Wonisch, A. and Raschi, A. 2004. Effects of sulphurous gases in two CO2 springs on total sulphur and thiols in acorns and oak seedlings. Atmospheric Environment 38: 3775-3780.

Hallgren, J.-E. 1984. Photosynthetic gas exchange in leaves affected by air pollutants. In: Koziol, M.J. and Whatley, F.R. (Eds.). Gaseous Air Pollutants and Plant Metabolism. Butterworths, London, UK, pp. 147-159.

Izrael, Yu.A., Gytarsky, M.L., Karaban, R.T., Lelyakin, A.L. and Nazarov, I.M. 2002. Consequences of climate change for forestry and carbon dioxide sink in Russian forests. Isvestiya, Atmospheric and Oceanic Physics 38: S84-S98.

Schulte, M., Raiesi, F.G., Papke, H., Butterbach-Bahl, K., van Breemen, N. and Rennenberg, H. 1999. CO2 concentration and atmospheric trace gas mixing ratio around natural CO2 vents in different Mediterranean forests in central Italy. In: Raschi, A., Vaccori, F.P. and Miglietta, F. (Eds.). Ecosystem Response to CO2: The Maple Project Results. European Communities, Brussels, Belgium, pp. 168-188.

Stylinski, C.D., Oechel, W.C., Gamon, J.A., Tissue, D.T., Miglietta, F. and Raschi, A. 2000. Effects of lifelong [CO2] enrichment on carboxylation and light utilization of Quercus pubescens Willd. examined with gas exchange, biochemistry and optical techniques. Plant, Cell and Environment 23: 1353-1362.

Tognetti,R., Johnson, J.D., Michelozzi, M. and Raschi, A. 1998. Response of foliar metabolism in mature trees of Quercus pubescens and Quercus ilex to long-term elevated CO2. Environmental and Experimental Botany 39: 233-245.