It is well known that plants photosynthesize and use carbon dioxide to produce energy and oxygen. But did you know that different photosynthetic pathways might have a huge impact on how plants will react to climate change?

This is all based around the inefficiency of standard, so called C₃ photosynthesis. Because photosynthesis evolved during a period of higher atmospheric CO₂, the main enzyme responsible for turning CO₂ into sugars, Rubisco, produces an intermediate product that likes reacting with oxygen (Andrews & Lorimer, 1987). This reaction, called photorespiration, releases CO₂ again rather than turning it into a sugar and means it has no useful function. This wastes enzyme capacity and energy (ATP) that could be used for photosynthesis (Ehleringer , et al., 1991). This whole problem makes you think that surely nature must have found a way around this inefficiency.

And in fact it did: At a time of lower CO₂ concentrations in the Earth’s history, flowering plants evolved a chemical pathway that increases the CO₂ concentration within the cells of leafs through an intermediate step involving 4-carbon acids and increases Rubisco efficiency as well as water and nitrogen use (Ehleringer, et al., 1991)! These C₄ plants dominate tropical and temperate grasslands and also include crops such as maize and sugar cane (Paul & Elmore, 1984), but only contribute about 20% of global primary production (Wand, et al., 1999).

However, climate change and increasing CO₂ levels in the atmosphere from fossil fuel emissions might render this current advantage worthless. While low CO₂ favours C₄ plants, high CO₂ levels increase the biomass of C₃ plants (Pearcy & Ehleringer, 1984). And while increasing temperature further reduces the CO₂ uptake of Rubisco in C₃ plants, doubling ambient CO₂ concentrations, something predicted to happen in the next 50 to 100 years, would cut photorespiration in half (Ehleringer, et al., 1991). C₄ photosynthesis on the other hand might reach a saturation point above ambient CO₂ levels and generally has a lower maximum production due to energy demand during the additional chemical pathway (Ehleringer, et al., 1991).

As with all processes in nature, this viewpoint may be too simplistic. Increasing CO₂ concentration also results in a positive response in C₄ plants, especially in conditions where they are already common (Sage & Kubien, 2003). Additionally, rising temperatures may favour C₄ plants and their higher nitrogen use efficiency might reduce effects of future nutrient limitations coupled with high CO₂ levels (Sage & Kubien, 2003). Generally, C₄ plants are predicted to be unaffected by environmental stress, while the stimulation of C₃ photosynthesis under increased CO₂ is reduced under stress, especially nutrient limitation (Wand, et al., 2003).

It is obvious that C₃ and C₄ plants differ in their responses to climate change and will prosper in different parts of the globe with major potential influences on the distribution of each photosynthetic type. But overall, differences in this specific reaction are not the only reason for these distinct responses and further research must show how we can use these to counteract effects of climate change.

 

References
Andrews, T. J., & Lorimer, G. H. (1987). The Biochemistry of Plants, edited by MD Hatch & NK Bordman.
Bloom, A. (2009). As carbon dioxide rises, food quality will decline without careful nitrogen management. California Agriculture, 63(2), 67-72.
Ehleringer, J. R., Sage, R. F., Flanagan, L. B., & Pearcy, R. W. (1991). Climate change and the evolution of C₄ photosynthesis. Trends in Ecology & Evolution, 6(3), 95-99.
Paul, R., & Elmore, C. D. (1984). Weeds and the C4 syndrome. Weeds Today, 15(1), 3-4.
Pearcy, R. W., & Ehleringer, J. (1984). Comparative ecophysiology of C₃ and C₄ plants. Plant, Cell & Environment, 7(1), 1-13.
Sage, R. F., & Kubien, D. S. (2003). Quo vadis C₄? An ecophysiological perspective on global change and the future of C4 plants. Photosynthesis Research, 77(2-3), 209-225.
Wand, S. J., Midgley, G., Jones, M. H., & 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(6), 723-741.

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