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First, What’s a C4 Plant?

I could answer this question here, but instead I’ll refer you to an excellent introduction provided by Dr. Colin Osborne. Or you can read the Wikipedia entry. (I’ll wait until you come back….)

OK….now you know that C4 plants – such as the crops maize, sorghum, and sugarcane – add a preliminary step to regular old C3 photosynthesis in order to increase the effective CO2 concentration for the enzyme RuBisCo. This results in much less photorespiration in C4 plants, which significantly increases their photosynthetic efficiency. Thus, in environments that promote photorespiration (e.g., hot, arid, and/or saline), C4 plants seemingly have a distinct selective advantage over most C3 plants.

Where Did C4 plants Come From?

Although C3 land plants have existed for nearly 500 million years, C4 plants didn’t arise until about 25 to 35 million years ago.

Why?

It’s probably because when, for a number of reasons, Earth’s atmospheric CO2 decreased to the point where photorespiration – which reduces photosynthetic efficiency – became a significant issue for plants, especially in hot, arid environments.

grassland.jpgThis limitation to photosynthetic productivity was reduced through the convergent evolution of C4 photosynthesis in nearly 50 independent flowering plant lineages.

This happened independently so many times likely because many C3 plants may already have had C4-type photosynthesis occurring in their stems (e.g., see Ref. 1 below).

The first C4 plants were probably grasses (monocots), followed several millions of years later by C4 dicots. Today, grasses represent about 2/3 of the roughly 7,500 species of C4 plants extant, with the rest split about evenly between dicots and sedges.

Despite the fact that C4 plants make up only about 3% of plant species, they account for nearly 25% of terrestrial photosynthesis (see Refs. 2 & 3 below). “C4 grasses and sedges dominate nearly all grasslands in the tropics, subtropics and warm temperate zones, and are major representatives of arid landscapes from the temperate zones to the tropics.” (from: Ref. 2)

The evolution of grasses using C4 photosynthesis and their sudden rise to ecological dominance 3 to 8 million years ago is among the most dramatic examples of biome assembly in the geological record. A growing body of work suggests that the patterns and drivers of C4 grassland expansion were considerably more complex than originally assumed.” (from: Ref. 3)

Considering the Increasing Levels of Atmospheric CO2, Whither C4 Plants?

smokestack.jpg Fast forward to today, with the increasing levels of atmospheric carbon dioxide, thanks mainly to the burning of fossil fuels.

Are C4 plants losing their CO2 advantage over C3 plants? And with atmospheric CO2 levels projected to double or even triple within the next 100 years, could C4 plants eventually disappear from the landscape? And what about C4 crop plants such as maize that constitute a major source of food and fuel?

Though C3 crop plants may benefit from increased atmospheric CO2, C4 crop plants will likely not benefit much, if at all. (e.g., see Ref. 4)

And what about native plant communities?

Although there is some evidence that increased CO2 may promote the displacement of some C4 grasses by C3 dicots on some rangelands, a great deal of uncertainty currently exists regarding the effects of CO2 alone. This is because of the multiple environmental effects (such as increased heat and drought) that accompany increasing levels of atmospheric CO2. (e.g., see Ref. 5)

Bottom line: Though C4 plants likely arose as a result of decreased levels of atmospheric CO2, their fate is very uncertain in the face of the increasing levels of CO2 that will likely occur in the centuries to come.

References

1. Hibberd, J.M. and Quick, W.P. (2002) “Characteristics of C4 photosynthesis in stems and petioles of C3 flowering plants.” Nature vol. 415, pp.451-454. (Abstract)

2. Sage, R.F. (2004) “The evolution of C4 photosynthesis.” New Phytologist vol. 161, pp. 341–370. (Abstract)

3. Edwards, E.J., Osborne, C.P., Strömberg, C.A.E., Smith, S.A., and C4 Grasses Consortium. (2010) “The origins of C4 grasslands: Integrating evolutionary and ecosystem science.” Science vol. 328, pp. 587-591. (Abstract)

4. Leakey, A.D.B. (2009) “Rising atmospheric carbon dioxide concentration and the future of C4 crops for food and fuel.” Proceedings of the Royal Society B vol. 276, pp. 2333-2343. (Abstract)

5. Bradley, B.A., Blumenthal, D.A., Wilcove, D.S., and Ziska, L.H. (2010) “Predicting plant invasions in an era of global change.” Trends in Ecology & Evolution vol. 25, pp. 310-318. (Abstract)

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