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 in spite of the apparent saturation of rubisco, atmospheric CO2 enrichment still enhances photosynthesis in C4 species; and this phenomenon has contributed to significant CO2-induced biomass increases in various types of C4 plants. In addition, because elevated CO2 nearly always reduces stomatal conductances and, hence, transpirational water losses in C4 plants, such vegetation typically exhibits increases in water-use efficiency in response to atmospheric CO2 enrichment. In this summary, we thus review the effects of elevated CO2 on parameters influencing water-use efficiency in C4 plants.
In the study of Maroco et al. (1999), Zea mays plants growing at an atmospheric CO2 concentration of 1100 ppm exhibited stomatal conductances that were 71% lower than rates displayed by control plants growing in air containing 350 ppm CO2. Likewise, Volin et al. (1998) reported that stomatal conductances in two C4 grasses grown at twice-ambient CO2 concentrations were significantly lower than those displayed by their respective controls. Moreover, in an open-top chamber study conducted on a tallgrass prairie, Adams et al. (2000) noted that a doubling of the atmospheric CO2 concentration caused consistent reductions in stomatal conductances and transpirational water losses in the dominant C4 grass (Andropogon gerardii), thus contributing to significant increases in water-use efficiency.
Similar CO2-induced increases in water-use efficiency have been quantified by Conley et al. (2001), who reported that Sorghum bicolor grown in FACE plots receiving 570 ppm CO2 exhibited water-use efficiencies that were 9 and 19% greater than those exhibited by ambiently-growing plants under well-watered and water-stressed conditions, respectively. In addition, in the previously-mentioned study of Maroco et al. (1999), Zea mays grown at 1100 ppm displayed a water-use efficiency that was 225% greater than that displayed by control plants grown at 350 ppm CO2.
In summary, it is clear that C4 plants do indeed respond positively to increases in the air's CO2 concentration by exhibiting reduced stomatal conductances and transpirational water losses, which contribute to increases in water-use efficiency. Hence, knowledgeable researchers are suggesting that the long-held view that C4 plants will not be benefited by elevated concentrations of atmospheric CO2 needs to be replaced with this more correct assessment (Wand et al., 1999; Zhu et al., 1999). Clearly, as the atmospheric CO2 concentration increases, most C4 plants will almost certainly display increases in water-use efficiency, which should allow them to better deal with conditions of water stress. Consequently, this phenomenon should allow plants of the future to expand their ranges into areas where they currently cannot survive due to limited soil moisture availability, thereby contributing to a great "greening of the globe."
References
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.
Conley, M.M., Kimball, B.A., Brooks, T.J., Pinter Jr., P.J., Hunsaker, D.J., Wall, G.W., Adams, N.R., LaMorte, R.L., Matthias, A.D., Thompson, T.L., Leavitt, S.W., Ottman, M.J., Cousins, A.B. and Triggs, J.M. 2001. CO2 enrichment increases water-use efficiency in sorghum. New Phytologist 151: 407-412.
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.
Volin, J.C., Reich, P.B. and Givnish, T.J. 1998. Elevated carbon dioxide ameliorates the effects of ozone on photosynthesis and growth: species respond similarly regardless of photosynthetic pathway or plant functional group. New Phytologist 138: 315-325.
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.