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C4 Plants (Biomass) -- Summary
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 scientists 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 can, and does, elicit substantial photosynthetic enhancements in C4 species.  Consequently, this phenomenon contributes to significant CO2-induced biomass increases in various types of C4 plants.

In agricultural species, for example, a tripling of the atmospheric CO2 content has been shown to increase the biomass of Zea mays by 20% (Maroco et al., 1999).  In addition, in a FACE experiment in which certain plots received an additional 200 ppm of CO2, Ottman et al. (2001) reported that the extra CO2 enhanced the yield of Sorghum bicolor by 15% under low soil moisture conditions.  Likewise, Watling and Press (1997) noted that a doubling of the atmospheric CO2 concentration boosted biomass production in Sorghum bicolor by 36%.

In C4 grasses, atmospheric CO2 enrichment has also been shown to be beneficial.  Ziska et al. (1999) reported that twice-ambient CO2 concentrations enhanced total biomass in Flaveria trinervia by 50%.  Similarly, a mere 150-ppm increase in the air's CO2 content boosted shoot biomass in Andropogon gerardii and Schizachyrium scoparium by 57% (Derner et al., 2001).  Under conditions of high soil nitrogen, Ghannoum and Conroy (1998) reported that a doubling of the atmospheric CO2 concentration enhanced total dry mass of Panicum coloratum and Panicum antidotale by 27%.  In addition, Seneweera et al. (2001) demonstrated that the dry mass of Panicum coloratum plants grown at an atmospheric CO2 concentration of 1000 ppm was 44 and 70% greater than that displayed by control plants grown in ambient air under well-watered and water-stressed conditions, respectively.  Lastly, in analyzing over 165 peer-reviewed scientific journal articles dealing with pasture and rangeland responses to global climate change parameters, Campbell et al. (2000) concluded that the "growth of C4 species is about as responsive to CO2 concentration as are C3 species when water supply restricts growth, as is usual in grasslands containing C4 species."

Finally, with respect to herbaceous C4 dicotyledonous species, Ziska and Bunce (1999) grew Amaranthus retroflexus at twice-ambient CO2 concentrations for about three weeks, finding that elevated CO2 enhanced its dry mass by 21%.

In summary, it is clear that C4 plants do indeed respond positively to increases in the atmosphere's CO2 concentration by exhibiting increases in biomass production, as is being noted more and more frequently by knowledgeable researchers (Wand et al., 1999; Zhu et al., 1999).  Thus, as the air's CO2 content continues to rise, C4 plants will likely display increases in growth in both agricultural fields and unmanaged natural grasslands.


Campbell, B.D., Stafford Smith, D.M., Ash, A.J., Fuhrer, J., Gifford, R.M., Hiernaux, P., Howden, S.M., Jones, M.B., Ludwig, J.A., Manderscheid, R., Morgan, J.A., Newton, P.C.D., Nosberger, J., Owensby, C.E., Soussana, J.F., Tuba, Z. and ZuoZhong, C.  2000.  A synthesis of recent global change research on pasture and rangeland production: reduced uncertainties and their management implications.  Agriculture, Ecosystems and Environment 82: 39-55.

Derner, J.D., Polley, H.W., Johnson, H.B. and Tischler, C.R.  2001.  Root system response of C4 grass seedlings to CO2 and soil water.  Plant and Soil 231: 97-104.

Ghannoum, O. and Conroy, J.P.  1998.  Nitrogen deficiency precludes a growth response to CO2 enrichment in C3 and C4 Panicum grasses.  Australian Journal of Plant Physiology 25: 627-636.

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: 1 15-125.

Ottman, M.J., Kimball, B.A., Pinter Jr., P.J., Wall, G.W., Vanderlip, R.L., Leavitt, S.W., LaMorte, R.L., Matthias, A.D. and Brooks, T.J.  2001.  Elevated CO2 increases sorghum biomass under drought conditions.  New Phytologist 150: 261-273.

Seneweera, S., Ghannoum, O. and Conroy, J.P.  2001.  Root and shoot factors contribute to the effect of drought on photosynthesis and growth of the C4 grass Panicum coloratum at elevated CO2 partial pressures.  Australian Journal of Plant Physiology 28: 451-460.

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.

Watling, J.R. and Press, M.C.  1997.  How is the relationship between the C4 cereal Sorghum bicolor and the C3 root hemi-parasites Striga hermonthica and Striga asiatica affected by elevated CO2Plant, Cell and Environment 20: 1292-1300.

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.