What was done
The authors grew spring wheat in FACE plots receiving 360 and 550 ppm atmospheric CO2.  In order to determine whether leaf position within a canopy affects photosynthetic acclimation to elevated CO2, net photosynthesis and photosynthetic protein data were obtained from leaves located at different vertical positions within the crop canopy.

What was learned
Elevated CO2 led to a certain degree of photosynthetic acclimation or down regulation by reducing leaf rubisco and nitrogen levels; and this phenomenon intensified with depth in the canopy.  However, the acclimation was not complete; and elevated CO2 still enhanced photosynthesis at all canopy levels relative to similarly located leaves in ambient air.  The authors state that this situation could only result from a CO2-induced "increased efficiency of leaf photosynthetic nitrogen use."

What it means
Acclimation of photosynthesis is not a negative phenomenon.  By reducing rubisco and leaf nitrogen, for example, greater allocations of nitrogen can be made to developing grains or fruits.  This additional nitrogen can then be used to support larger reproductive structures, thereby enabling them to produce and sustain the larger yields that are commonly reported for crops exposed to elevated levels of atmospheric CO2.