What was done
The authors grew cotton (Gossypium hirsutum L.) for 50 days in open-top chambers receiving atmospheric CO2 concentrations of 350, 525 and 700 ppm.  The plants were fertilized with three levels of nitrogen and subjected to either no O3 (ozone) or 1.5-times the ambient O3 level to determine the effects of elevated CO2, O3 and soil nitrogen on various leaf parameters in this woody crop species.

What was learned
In the absence of O3, elevated CO2 significantly increased leaf mass per unit area by 36 and 51% in plants subjected to 525 and 700 ppm CO2, respectively.  In the presence of O3, elevated CO2 nearly eliminated the negative effects of this gas on leaf mass.  Leaf mass per unit area, for example, was 25% lower in plants grown at ambient CO2 and elevated O3 than it was for plants grown at ambient CO2 alone.  However, leaf mass per unit area was only 5% less for plants grown at 700 ppm CO2 and elevated O3 than it was for plants grown at 700 ppm CO2 alone.

Similar results were observed for leaf starch concentrations, which in the absence of O3 were significantly increased by 39 and 59% in leaves grown at 525 and 700 ppm CO2, respectively.  Elevated O3 reduced foliar starch concentrations by 48% for plants grown simultaneously at ambient CO2 concentrations.  However, elevated O3 only reduced starch concentrations by a mere 7% when plants were subjected to an atmospheric CO2 concentration of 700 ppm.

What it means
As the air's CO2 content rises, cotton plants will likely experience reduced amounts of ozone-induced damage.  Indeed, the results of this paper demonstrate that elevated atmospheric CO2 concentrations ameliorate the negative effects of elevated O3 concentrations on leaf mass and foliar starch concentrations.  Thus, with healthier leaves and more starch available to them, cotton plants will likely be able to utilize additional energy to enhance their productivity and produce greater yields.