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CO2, Global Warming and Sugarcane: Prospects for the Future
Volume 12, Number 52: 30 December 2009

"C4 plants," in the words of Vu and Allen (2009), "represent fewer than 4% of all angiosperm species," yet "their ecological and economic significance is substantial." On a global basis, for example, they write that "up to one-third of terrestrial productivity is provided by C4 plants," citing Cerling et al. (1997), Ghannoum et al. (1997) and Brown et al. (2005). In this regard, they note that "in many tropical regions, the food source is primarily based on C4 crops, among [which] maize, millet, sorghum and sugarcane are the most agriculturally important monocots in terms of production (Brown, 1999)," with "up to 75% of the world sugar production [being] provided by sugarcane (De Souza et al., 2008)." And they note that the emerging "use of sugarcane as a source for biofuel production has been highly recognized," citing Goldenberg (2007).

So what will happen to the productivity of this important crop as the air's CO2 content continues its upward climb, especially if global air temperatures rise along with it, for whatever reason?

Historically, C4 crops have been thought to be relatively unresponsive to atmospheric CO2 enrichment, as they possess a CO2 concentrating mechanism that allows them to achieve a greater photosynthetic capacity than C3 plants at the current atmospheric CO2 concentration, particularly at high growth temperatures (Matsuoka et al., 2001). Thus, simple reasoning would suggest that C4 plants may be little benefited -- if at all -- in a CO2-enriched and warmer world of the future. As one may readily discern, however, simply by looking up C4 Plants (Biomass, Photosynthesis and Water Use Efficiency) in our Subject Index, this view is not correct. And in the case of sugarcane, as the research of Vu and Allen demonstrates, it is very incorrect, especially with respect to the most important measure of sugarcane's economic value: stem juice production.

The two researchers with the USDA's Agricultural Research Service -- who hold joint appointments in the Agronomy Department of the University of Florida (USA) -- grew two cultivars of sugarcane (Saccharum officinarum) for a period of three months in paired-companion, temperature-gradient, sunlit greenhouses under daytime CO2 concentrations of 360 and 720 ppm and air temperatures of 1.5C (near ambient) and 6.0C higher than outside ambient temperature, after which they measured a number of different plant properties.

"On a main stem basis," in the words of Vu and Allen, "leaf area, leaf dry weight, stem dry weight and stem juice volume were increased by growth at doubled CO2 [as well as at] high temperature," and they say that these increases were even greater under the combination of doubled CO2/high temperature, with plants grown under what climate alarmists would call these extreme conditions averaging "50%, 26%, 84% and 124% greater leaf area, leaf dry weight, stem dry weight and stem juice volume, respectively, compared with plants grown at [the] ambient CO2/near-ambient temperature combination." In addition, they say that "plants grown at [the] doubled CO2/high temperature combination were 2- to 3-fold higher in stem soluble solids than those at [the] ambient CO2/near-ambient temperature combination."

Consequently, as Vu and Allen conclude -- based on their research and that of many other scientists -- "sugarcane grown under predicted rising atmospheric CO2 and temperature in the future may use less water, utilize water more efficiently, and would perform better in sucrose production," which bodes well indeed for tropical-region agriculture, especially, as they note, "with the worldwide continued increase in demand for sugarcane as a source of food and biofuel."

Last of all, they add that significant "improvements in stem sucrose and biomass through classical breeding and/or new biotechnology" may also be achieved; and, hence, they say that "studies to identify the cultivars with high efficiency in water use and stem sucrose production under future changes in CO2 and climate are of great importance and should be initiated and explored."

Working hand-in-hand with the benefits provided by the ongoing rise in the air's CO2 content, therefore, as well as those provided by the possibility of still higher air temperatures to come, we may yet be able to meet the increasing food needs of our expanding numbers without the taking of vast amounts of land and freshwater resources from what remains of earth's natural ecosystems.

Sherwood, Keith and Craig Idso

References
Brown, N.J., Parsley, K. and Hibberd, J.M. 2005. The future of C4 research - maize, Flaveria or Cleome? Trends in Plant Science 10: 215-221.

Brown, R.H. 1999. Agronomic implications of C4 photosynthesis. In Sage, R.F. and Monson, R.K. (Eds.) C4 Plant Biology. Academic Press, San Diego, California, USA, p. 473-507.

Cerling, T.E., Harris, J.M., MacFadden, B.J., Leakey, M.G., Quade, J., Eisenmann, V. et al. 1997. Global vegetation change through the Miocene/Pliocene boundary. Nature 389: 153-158.

De Souza, A.P., Gaspar, M., Da Silva, E.A., Ulian, U.C., Waclawovsky, A.J., Nishiyama Jr., M.Y. et al. 2008. Elevated CO2 increases photosynthesis, biomass and productivity, and modifies gene expression in sugarcane. Plant, Cell and Environment 31: 1116-1127.

Ghannoum, O., von Caemmerer, S., Barlow, E.W.R. and Conroy, J.P. 1997. The effect of CO2 enrichment and irradiance on the growth, morphology and gas exchange of a C3 (Panicum laxum) and a C4 (Panicum antodotale) grass. Australian Journal of Plant Physiology 24: 227-237.

Goldenberg, J. 2007. Ethanol for a sustainable energy future. Science 315: 808-810.

Matsuoka, M., Furbank, R.T., Fukayama, H. and Miyao, M. 2001. Molecular engineering of C4 photosynthesis. Annual Review of Plant Physiology and Plant Molecular Biology 52: 297-314.

Vu, J.C.V. and Allen Jr., L.H. 2009. Stem juice production of the C4 sugarcane (Saccharum officinarum) is enhanced by growth at double-ambient CO2 and high temperature. Journal of Plant Physiology 166: 1141-1151.