What was done
The authors grew 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 alone and in combination with 60 ppb SO2 to study the interactive effects of elevated CO2 and this major air pollutant on the growth and yield of this important crop.

What was learned
Exposure to elevated CO2 significantly increased photosynthetic rates by 58 and 48% in M234 and HP1209, respectively.  In contrast, fumigation with elevated SO2 did not significantly impact 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 also displayed an approximate 20% reduction in stomatal conductance, while those grown in elevated SO2 exhibited an average increase of 15%.  When exposed simultaneously to both gases, however, plants displayed an average 11% reduction in stomatal conductance.  Consequently, this phenomenon contributed to an approximate 32% increase in water-use efficiency for plants simultaneously exposed to both gases, whereas those exposed to elevated SO2 alone displayed an average decrease in water-use efficiency of 16%.

Last of all, exposure to elevated SO2 caused an average 13% decrease in foliar protein concentrations in both cultivars.  However, when plants were concomitantly 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.

What it means
As the air's CO2 content continues to rise, it will likely allow these wheat cultivars and others to experience less stress and growth reductions as a consequence of SO2 pollution.  Indeed, the current 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.  In addition, since SO2-induced reductions in plant water-use efficiency were essentially eliminated by concurrent exposure to elevated CO2, these cultivars should be able to grow better in areas with limited water availability or in areas close to industrial complexes emitting large quantities of SO2.  Also, wheat plants growing in SO2-polluted air should not suffer as large reductions in foliar protein content in a future high-CO2 world as they do currently.