What was done
The authors grew Citrus madurensis Loureiro (Calamondin) cuttings for three months in controlled environment chambers receiving atmospheric CO2 concentrations of 300, 450, 600, 750 and 900 ppm to study the effects of elevated CO2 on various leaf functions and properties.

What was learned
At 600 ppm CO2, the photosynthetic rates of completely expanded leaves were more than 300% greater than those of similar leaves exposed to 300 ppm CO2.  In addition, old leaves exposed to 600 ppm CO2 exhibited photosynthetic rates that were 90% greater than those observed in old leaves growing on cuttings maintained at 300 ppm CO2.  Interestingly, elevated CO2 did not affect leaf chlorophyll content, regardless of leaf age.  However, leaf nitrogen content decreased with increasing atmospheric CO2 concentration and leaf age.  Thus, the leaf chlorophyll to nitrogen ratio increased with atmospheric CO2 enrichment and leaf age, indicating that older leaves adapted to elevated CO2 conditions by increasing their photosynthetic nitrogen-use efficiency to a greater extent than younger leaves.

What it means
As the air's CO2 content increases, Citrus trees will almost assuredly exhibit enhanced photosynthetic rates that will likely lead to greater biomass and fruit production.  In addition, elevated CO2 concentrations will likely enhance photosynthetic nitrogen-use efficiency in Citrus, significantly reducing what would otherwise be a much greater need for nitrogen inputs in orchard ecosystems.