As the CO2 content of the air progressively declined millions of years ago, certain plants evolved specialized biochemical pathways and anatomical adaptations that enabled them to increase their intracellular CO2 concentration at the site of its fixation, which allowed the primary carboxylating enzyme rubisco to function more efficiently. The CO2 concentrating mechanism possessed by these C4 plants operates by sequentially reducing CO2 into carbohydrates within two different sets of cells. The initial reduction of CO2 into a four-carbon sugar is done within standard photosynthetic parenchyma cells by the enzyme PEP carboxylase. Then, the four-carbon sugar is transported to specialized bundle sheath cells where it is decarboxylated, increasing the plant's intercellular CO2 concentration, after which it is reduced back into a carbohydrate, but this time by rubisco.
Because this CO2-concentrating mechanism is believed to saturate rubisco, some researchers have suggested that C4 plants will not respond positively to rising levels of atmospheric CO2. However, it has been shown that despite the apparent saturation of rubisco, atmospheric CO2 enrichment still elicits substantial photosynthetic enhancements in various C4 plants.
With respect to grasses, Fritschi et al. (1999) reported that net photosynthetic rates were 22% greater for bahiagrass when it was grown at an atmospheric CO2 concentration of 640 ppm as opposed to 365 ppm. Similarly, Leonardos and Grodzinski (2000) found that a 540-ppm increase in the air's CO2 concentration boosted rates of net photosynthesis in Panicum and Flaveria genera by an average of 13%. Ziska et al. (1999) further noted that twice-ambient CO2 concentrations increased photosynthesis in species from these genera by 12 and 19% under moderate and high light intensity, respectively; and in a two-year open-top chamber study conducted on a tallgrass prairie, a 350-ppm increase in the air's CO2 content significantly increased rates of net photosynthesis in CO2-enriched plots, but only during the relatively dry year of the study (Adams et al., 2000). Finally, Wand et al. (1999) conducted a massive review of the scientific literature published between 1980 and 1997, wherein they determined that a doubling of the air's CO2 content increased the photosynthetic rates of C4 grasses by an average of 25%.
With respect to other types of C4 plants, a doubling of the atmospheric CO2 concentration enhanced rates of photosynthesis in Amaranthus retroflexus (an herbaceous C4 dicotyledonous species) by 13 to 20% (Ziska and Bunce, 1999; Ward et al., 1999). As for agricultural species, a 200-ppm increase in the air's CO2 content enhanced photosynthetic rates in Sorghum bicolor by 15% (Cousins et al., 2001), while a tripling of the atmospheric CO2 concentration increased photosynthetic rates in Zea mays by 15% (Maroco et al., 1999).
In summary, it is clear that C4 plants can, and do, respond positively to increases in the air's CO2 concentration by exhibiting enhanced rates of photosynthesis. Knowledgeable researchers are thus suggesting that the long-held but erroneous view that C4 plants will not respond to elevated atmospheric CO2 concentrations be replaced with this more correct assessment of the situation (Wand et al., 1999; Zhu et al., 1999).
Adams, N.R., Owensby, C.E. and Ham, J.M. 2000. The effect of CO2 enrichment on leaf photosynthetic rates and instantaneous water use efficiency of Andropogon gerardii in the tallgrass prairie. Photosynthesis Research 65: 121-129.
Cousins, A.B., Adam, N.R., Wall, G.W., Kimball, B.A., Pinter Jr., P.J., Leavitt, S.W., LaMorte, R.L., Matthias, A.D., Ottman, M.J., Thompson, T.L. and Webber, A.N. 2001. Reduced photorespiration and increased energy-use efficiency in young CO2-enriched sorghum leaves. New Phytologist 150: 275-284.
Fritschi, F.B., Boote, K.J., Sollenberger, L.E., Allen, Jr. L.H. and Sinclair, T.R. 1999. Carbon dioxide and temperature effects on forage establishment: photosynthesis and biomass production. Global Change Biology 5: 441-453.
Leonardos, E.D. and Grodzinski, B. 2000. Photosynthesis, immediate export and carbon partitioning in source leaves of C3, C3-C4 intermediate, and C4 Panicum and Flaveria species at ambient and elevated CO2 levels. Plant, Cell and Environment 23: 839-851.
Maroco, J.P., Edwards, G.E. and Ku, M.S.B. 1999. Photosynthetic acclimation of maize to growth under elevated levels of carbon dioxide. Planta 210: 115-125.
Wand, S.J.E., Midgley, G.F., Jones, M.H. and Curtis, P.S. 1999. Responses of wild C4 and C3 grass (Poaceae) species to elevated atmospheric CO2 concentration: a meta-analytic test of current theories and perceptions. Global Change Biology 5: 723-741.
Ward, J.K., Tissue, D.T., Thomas, R.B. and Strain, B.R. 1999. Comparative responses of model C3 and C4 plants to drought in low and elevated CO2. Global Change Biology 5: 857-867.
Zhu, J., Goldstein, G. and Bartholomew, D.P. 1999. Gas exchange and carbon isotope composition of Ananas comosus in response to elevated CO2 and temperature. Plant, Cell and Environment 22: 999-1007.
Ziska, L.H. and Bunce, J.A. 1999. Effect of elevated carbon dioxide concentration at night on the growth and gas exchange of selected C4 species. Australian Journal of Plant Physiology 26: 71-77.
Ziska, L.H., Sicher, R.C. and Bunce, J.A. 1999. The impact of elevated carbon dioxide on the growth and gas exchange of three C4 species differing in CO2 leak rates. Physiologia Plantarum 105: 74-80.