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Effects of Elevated CO2 on Respiration in Prickly Pear Cactus
Reference
Gomez-Casanovas, N., Blanc-Betes, E., Gonzalez-Meler, M.A. and Azcon-Bieto, J. 2007. Changes in respiratory mitochondrial machinery and cytochrome and alternative pathway activities in response to energy demand underlie the acclimation of respiration to elevated CO2 in the invasive Opuntia ficus-indica. Plant Physiology 145: 49-61.

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 (20C and 30C)." 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.

Reviewed 23 January 2008