Background
Plants grown in CO2-enriched environments sometimes exhibit some degree of photosynthetic acclimation or down regulation, which is typically characterized by a reduction in their initial CO2-enhanced rates of photosynthesis that results from a gradual decrease in the activity and/or amount of rubisco, which is the primary plant carboxylating enzyme.

What was done
Diurnal changes in light-saturated net photosynthesis (Pn) rate under both ambient and elevated atmospheric CO2 and/or ozone (O3) concentrations were measured over wide ranges of stomatal conductance, water potential, intercellular CO2, leaf temperature, and vapor pressure difference between leaf and air in two clones (271 and 42E) of quaking aspen (Populus tremuloides Michx.) trees that differed in their sensitivity to ozone and had been growing at the Aspen FACE site near Rhinelander, Wisconsin (USA) for seven to eight years.

What was learned
Kets et al. report that Pn was typically enhanced by 33-46% in the CO2-enriched treatments over the course of their study, and that there was a small increase in leaf chlorophyll concentration as well.

What it means
Noting that "previous Aspen FACE studies have reported 25-36% increases in Pn (Noormets et al., 2001; Takeuchi et al., 2001; Sharma et al., 2003; Ellsworth et al., 2004)," the six scientists emphasize that the aerial fertilization effect of atmospheric CO2 enrichment on Pn observed in their study "has rather been increasing in time than decreasing," stating that this phenomenon may be caused by the "slight but significant increase in leaf chlorophyll content per leaf area, which is rather positive acclimation in photosynthetic apparatus than negative acclimation [italics added]," in support of which conclusion they also cite the studies of Centritto and Jarvis (1999) and Eichelmann et al. (2004). Hence, their experimental persistence demonstrates that some of the benefits of elevated atmospheric CO2 concentrations may actually increase with the passage of time.

References
Centritto, M. and Jarvis, P.G. 1999. Long-term effects of elevated carbon dioxide concentration and provenance on four clones of Sitka spruce (Picea sitchensis). II. Photosynthetic capacity and nitrogen use efficiency. Tree Physiology 19: 807-814.

Eichelmann, H., Oja, V., Rasulov, B., Padu, E., Bichele, I., Pettai, H., Mols, T., Kasparova, I., Vapaavuori, E. and Laisk, A. 2004. Photosynthetic parameters of birch (Betula pendula Roth) leaves growing in normal and in CO2- and O3-enriched atmospheres. Plant, Cell and Environment 27: 479-495.

Ellsworth, D.S., Reich, P.B., Naumburg, E.S., Koch, G.W., Kubiske, M.E. and Smith, S.D. 2004. Photosynthesis, carboxylation and leaf nitrogen responses of 16 species to elevated pCO2 across four free-air CO2 enrichment experiments in forest, grassland and desert. Global Change Biology 10: 2121-2138.

Noormets, A., Sober, A., Pell, E.J., Dickson, R.E., Podila, G.K., Sober, J., Isebrands, J.G. and Karnosky, D.F. 2001. Stomatal and non-stomatal limitation to photosynthesis in two trembling aspen (Populus tremuloides Michx.) clones exposed to elevated CO2 and O3. Plant, Cell and Environment 24: 327-336.

Sharma, P., Sober, A., Sober, J., Podila, G.K., Kubiske, M.E., Mattson, W.J., Isebrands, J.G. and Karnosky, D.F. 2003. Moderation of CO2-induced gas exchange responses by elevated tropospheric O3 in trembling aspen and sugar maple. Ekologia 22 (S1): 304-317.

Takeuchi, Y., Kubiske, M.E., Isebrands, J.G., Pregitzer, K.S., Hendrey, G. and Karnosky, D.F. 2001. Photosynthesis, light and nitrogen relationships in a young deciduous forest canopy under open-air CO2 enrichment. Plant, Cell and Environment 24: 1257-1268.