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 of the fact that this CO2-concentrating mechanism is believed to saturate rubisco, some researchers have suggested that C4 plants will not respond to rising levels of atmospheric CO2. However, it has been shown that despite the apparent saturation of rubisco, atmospheric CO2 enrichment can, and does, elicit substantial photosynthetic enhancements in C4 plants. Fritschi et al. (1999), for example, recently reported that net photosynthetic rates were 22% greater for bahiagrass when it was grown at an atmospheric CO2 concentration of 640 ppm instead of 365 ppm. In addition, Wand et al. (1999) conducted a massive review of the scientific literature published between 1980 and 1997, determining that a doubling of the air's CO2 content increased the photosynthetic rates of C4 grasses by an average of 25%.
Besides increasing photosynthetic rates, elevated CO2 can also enhance biomass production in C4 plants (Fritschi et al., 1999; Owensby et al., 1999). When exposed to an atmospheric CO2 concentration of 700 ppm, for example, a desert grass produced 25% more biomass than it did at ambient CO2 (BassiriRad et al., 1998). Similarly, in a study involving a perennial grass species, fumigation with 1000 ppm CO2 led to stem dry weights that were more than 50% greater than those of control plants exposed to 350 ppm CO2 (Seneweera et al., 1998). And in the comprehensive review of Wand et al. (1999), a doubling of the CO2 content of the air resulted in an average 33% increase in C4 grass biomass.
On another note, C4 plants typically exhibit less transpirational water loss than most C3 plants, due to the physical shape of their stomatal guard cells. Thus, because C4 plants are already relatively efficient water users at ambient CO2 concentrations, it has been suggested that atmospheric CO2 enrichment may not further enhance their water-use efficiency. However, several recent studies demonstrate that C4 plants do indeed increase their water-use efficiency in response to atmospheric CO2 enrichment (Clark et al., 1999; LeCain and Morgan, 1998; Seneweera et al. 1998).
In summary, it is clear that C4 plants can, and do, respond positively to increases in the air's CO2 concentration. Hence, knowledgeable researchers are suggesting that the long-held view that C4 plants will not respond to elevated CO2 needs to be replaced with this more correct assessment (Wand et al., 1999; Zhu et al., 1999).
References
BassiriRad, H., Reynolds, J.F., Virginia, R.A. and Brunelle, M.H. 1998. Growth and root NO3- and PO43- uptake capacity of three desert species in response to atmospheric CO2 enrichment. Australian Journal of Plant Physiology 24: 353-358.
Clark, H., Newton, P.C.D. and Barker, D.J. 1999. Physiological and morphological responses to elevated CO2 and a soil moisture deficit of temperate pasture species growing in an established plant community. Journal of Experimental Botany 50: 233-242.
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.
LeCain, D.R. and Morgan, J.A. 1998. Growth, gas exchange, leaf nitrogen and carbohydrate concentrations in NAD-ME and NADP-ME C4 grasses grown in elevated CO2. Physiologia Plantarum 102: 297-306.
Owensby, C.E., Ham, J.M., Knapp, A.K. and Auen, L.M. 1999. Biomass production and species composition change in a tallgrass prairie ecosystem after long-term exposure to elevated atmospheric CO2. Global Change Biology 5: 497-506.
Seneweera, S.P., Ghannoum, O. and Conroy, J. 1998. High vapor pressure deficit and low soil water availability enhance shoot growth responses of a C4 grass (Panicum coloratum cv. Bambatsi) to CO2 enrichment. Australian Journal of Plant Physiology 25: 287-292.
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.
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.