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Effects of Elevated CO2 on Photosynthesis and Water Use in Two Grasses and Two Legumes of the Mediterranean Region
Roumet, C., Garnier, E., Suzor, H., Salager, J.-L. and Roy, J.  2000.  Short and long-term responses of whole-plant gas exchange to elevated CO2 in four herbaceous species.  Environmental and Experimental Botany 43: 155-169.

What was done
The authors grew seedlings of four Mediterranean herbaceous species -- two C3 grasses (one annual and one perennial species) and two nitrogen-fixing legumes (one annual and one perennial species) -- in pots placed within glasshouses fumigated with atmospheric CO2 concentrations of 350 and 700 ppm for six months to study the effects of elevated CO2 on their gas exchange properties.

What was learned
After six months of differential CO2 treatment, short-term exposure of ambiently-grown plants to an atmospheric CO2 concentration of 700 ppm increased their photosynthetic rates by approximately 35 and 20% for the grasses and legumes, respectively.  However, the long-term photosynthetic responses of most plants grown and measured at atmospheric CO2 concentrations of 700 ppm were not significantly different from those of control plants grown at ambient CO2, indicating that photosynthetic acclimation had occurred in these species.  Nevertheless, atmospheric CO2 enrichment significantly reduced the stomatal conductances of all species in both the short- and long-term, enabling them to maintain significant CO2-induced increases in plant water-use efficiency throughout the entire experiment.

In taking a closer look at photosynthetic acclimation, the authors noted that total nonstructural carbohydrates in the CO2-enriched grass leaves increased by an average of 51%, while elevated CO2 had no effect on total nonstructural carbohydrates in the leaves of the leguminous species.  In addition, elevated CO2 significantly reduced leaf nitrogen content by about 12% in the grasses, but had no effect on leaf nitrogen content in the legumes.  Thus, the authors concluded that photosynthetic acclimation in the grasses may have been triggered by CO2-induced reductions in foliar nitrogen and/or CO2-induced accumulation of total nonstructural carbohydrates.

What it means
As the air's CO2 content increases, these four Mediterranean herbaceous species will likely respond by simultaneously increasing their photosynthetic rates and decreasing their stomatal conductances.  It is possible that photosynthetic acclimation could occur in the grasses, but all of the species should exhibit increases in water-use efficiency, which should allow them to better cope with drought stress brought on by low levels of soil moisture.