What was done
The authors grew sunflowers in large controlled-environment chambers receiving ambient and twice-ambient concentrations of atmospheric CO2 to study the effects of elevated CO2 on canopy photosynthesis.

What was learned
Atmospheric CO2 enrichment increased photosynthetic rates of young upper canopy leaves by about 50% at high light intensities compared to rates measured on plants grown at ambient CO2.  When gas exchange rates of leaves from both CO2 treatments were measured at the same CO2 concentrations, it was determined that their photosynthetic capacities did not significantly differ from one another.  However, when measuring photosynthetic rates of older leaves located deeper within the canopy, it was found that elevated CO2 reduced their photosynthetic capacities by up to 40% relative to older leaves on control plants growing at ambient CO2.  Similarly, elevated CO2 had no effect on leaf rubisco content in the younger leaves composing the upper canopy, but significantly reduced it in older leaves located lower within the canopy.  It is interesting to note that although elevated CO2 had little effect on total leaf area, it did cause a shift in the distribution of leaf area to concentrate 30 to 40% more of it in the upper canopy where the CO2-induced photosynthetic stimulation was greatest.

What it means
As the CO2 content of the air continues to increase, sunflowers will likely increase their rates of photosynthesis and overall production of biomass.  Although photosynthetic acclimation will occur to some degree in leaves located lower on sunflower stems, a greater proportion of leaves will be distributed in the upper canopy and will likely exhibit no reductions in photosynthetic capacity.  Thus, greater carbohydrate production should result, which will likely increase marketable yields of sunflower products.