What was done
Swards of perennial ryegrass (Lolium perenne) were grown as a frequently-cut herbage crop in a FACE experiment employing low and high soil nitrogen supplies and air of either 360 or 600 ppm CO2 to determine how these variables influence photosynthetic acclimation.  Leaves were sampled one day prior to cutting and seven days later for carbohydrate, nitrogen, protein and gas exchange parameters to determine if decreasing the source capacity for carbohydrates (a result of cutting) would have an influence on photosynthetic acclimation.  The authors hypothesized that low soil nitrogen would exacerbate photosynthetic acclimation at elevated CO2 and that cutting would alleviate this phenomenon.

What was learned
Photosynthetic rates were approximately 35% greater for plants grown at elevated CO2, regardless of nitrogen supply or cutting.  In addition, plants grown at elevated CO2 and high nitrogen supply failed to exhibit any down regulation of rubisco both before and after cutting.  In contrast, plants grown at elevated CO2 and low soil nitrogen exhibited a 25% reduction in leaf rubisco before cutting; but this acclimation phenomenon was completely ameliorated after cutting.  These observations support the authors' hypothesis that nitrogen availability may be more important than CO2 concentration in causing photosynthetic acclimation.

Other measurements indicated that plants grown at elevated CO2 and high soil nitrogen exhibited no significant differences in leaf nonstructural carbohydrate content relative to plants grown at ambient CO2 and high soil nitrogen prior to cutting, but that leaf carbohydrate contents of plants grown at elevated CO2 and low soil nitrogen were almost twice as great as those of plants grown at ambient CO2 and low soil nitrogen prior to cutting.  Following cutting, however, which lowered the source:sink ratio of the plants, leaf carbohydrate contents significantly decreased, probably in response to their being used to repair cut leaves and sustain a suddenly relatively-greater carbohydrate sink.  These observations further suggest, according to the authors, that photosynthetic acclimation at low soil nitrogen results from the inability of plants to develop sinks that can utilize additional carbohydrates commonly produced under conditions of atmospheric CO2 enrichment.  Hence, photosynthetic acclimation appears to result from an indirect effect of low soil nitrogen on sink development, rather than from a direct effect of elevated CO2 on the leaf's photosynthetic apparatus.

What it means
As the concentration of CO2 in the air rises, photosynthetic acclimation may occur in plants growing at low soil nitrogen levels that typically prevent the development of strong plant sinks for carbohydrates.  Under such circumstances, carbohydrates may accumulate in leaves and create a feedback inhibition, or acclimation, of photosynthesis.  This phenomenon could result in decreases in leaf concentrations of nitrogen-rich rubisco.  This response, however, is desirable, because with less leaf rubisco, greater amounts of nitrogen can be mobilized to other areas of the plant to support increased growth and development.  And this sink growth, in turn, may actually reduce the degree of photosynthetic acclimation that was initially experienced.