Photosynthetic Acclimation to Elevated CO2 in Quaking Aspen Trees
Kets, K., Darbah, J.N.T., Sober, A., Riikonen, J., Sober, J. and Karnosky, D.F. 2010. Diurnal changes in photosynthetic parameters of Populus tremuloides, modulated by elevated concentrations of CO2 and/or O3 and daily climatic variation. Environmental Pollution 158: 1000-1007.
In the present study, the phenomenon of acclimation was investigated via diurnal changes in light-saturated net photosynthesis (Pn) rate under both ambient and elevated atmospheric CO2 and/or ozone (O3) concentrations 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.
According to authors Kets et al. (2010), Pn was typically enhanced by 33-46% in the CO2-enriched treatments over the course of their study, and there was a small increase in leaf chlorophyll concentration as well. 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.
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