What was done
The authors grew well watered and fertilized individual prickly pear cactus (Opuntia ficus-indica Miller) plants from single mature cladodes transplanted vertically into 9 to12-L pots (filled with a 2:1:1 peat:vermiculite:perlite mixture), which were located within controlled-environment chambers (at CO2 concentrations of 380 and 780 ppm) and greenhouses (at CO2 concentrations of 380 and 600 ppm) for periods of 4 and 9 months, while measuring a number of physical and physiological properties of the Crassulacean acid metabolism (CAM) plants.

What was learned
At the 4-month point of the growth-chamber study, first- and second-daughter cladode biomass production was enhanced by 19 and 37%, respectively, in the high-CO2 air, while root biomass was enhanced by 20%. Likewise, Gomez-Casanovas et al. report that at the 9-month point of the greenhouse study, the biomass of the first-daughter cladodes was enhanced by 20% in the high-CO2 air. They also report that "dark respiration rates expressed on a dry mass basis and measured at different development stages were reduced by 20% in cladode segments of first-daughter cladodes of O. ficus-indica plants grown at elevated CO2 when compared to ambient CO2-grown plants," and that "this reduction in respiration rates was also observed when rates were measured at two different measurement temperatures (20°C and 30°C)." What is more, they say that "the inhibitory long-term effect of elevated CO2 on the rate of respiration was consistent throughout the 9 months of the study, despite the 3-fold variation in respiration rates seen with tissue age," and they say that "the magnitude [of] the reduction of dark respiration rates observed in O. ficus-indica first-daughter cladodes grown and developed in elevated CO2 was similar to those seen in C3 plants exposed to elevated CO2."

What it means
The results of this study of a common CAM plant, which are harmonious with the results of other studies of the same plant (Cui et al., 1993; Drennan and Nobel, 2000), as well as with the results of studies of many C3 plants (Amthor, 1997; Drake et al., 1997; Gonsalez-Meler et al., 2004), suggest the best of both major "plant-process worlds" in a CO2-accreting atmosphere, such as is essentially assured for decades, if not centuries, to come: a CO2-induced increase in plant carbon capture via photosynthesis, and a CO2-induced decrease in plant carbon loss via respiration.

References
Amthor, J.S. 1997. Plant respiratory responses to elevated CO2 partial pressure. In: Allen, L.H., Kirkham, M.B., Olszyk, D.M. and Whitman, C.E., Eds. Advances in Carbon Dioxide Effects Research. American Society of Agronomy Special Publication, Madison, Wisconsin, USA, pp. 35-77.

Cui, M., Miller, P.M. and Nobel, P.S. 1993. CO2 exchange and growth of the Crassulacean Acid Metabolism plant Opuntia ficus-indica under elevated CO2 in open top chambers. Plant Physiology 103: 519-524.

Drake, B.G., Gonzalez-Meler, M.A. and Long, S.P. 1997. More efficient plants: a consequence of rising atmospheric CO2? Annual Reviews of Plant Physiology and Plant Molecular Biology 48: 609-639.

Drennan, P.M. and Nobel, P.S. 2000. Responses of CAM species to increasing atmospheric CO2. Plant, Cell and Environment 23: 767-781.

Gonzalez-Meler, M.A., Taneva, L. and Trueman, R.J. 2004. Plant respiration and elevated atmospheric CO2 concentration: cellular responses and global significance. Annals of Botany (London) 94: 647-656.