Most plants (including soybeans, rice, canola, and all trees) are C3 because they fix CO₂ first into a carbohydrate containing three carbon atoms. Corn, sorghum, and sugarcane belong to a special group of plants known as C4, so-called because they first fix CO₂ into a four-carbon carbohydrate during photosynthesis. On average, C4 crops are 60 percent more productive than C3 crops.

When crops are grown in elevated CO₂ that mimic future atmospheric conditions, research shows that C3 crops can become more productive while some experiments suggest that C4 crops would be no more productive in a higher CO₂ world.

"As scientists, we need to think several steps ahead to anticipate what the Earth will look like five to 30 years from now, and how we can design crops to perform well under those conditions," said Charles Pignon, a former postdoctoral researcher at Illinois. "We decided that a literature review and a retrospective analysis of biochemical limitations in photosynthesis would be able to give us some insight into why C4 crops might not respond and how we might alter this."

The literature review, published in Plant, Cell & Environment, was supported by Water Efficient Sorghum Technologies (WEST), a research project that aimed to develop bioenergy crops that produce more biomass with less water, with funding from the Advanced Research Projects Agency-Energy (ARPA-E).