C4 bioenergy crop research: striving to improve productivity under fluctuating light

One of the major challenges facing the world is how to provide sustainable energy to meet social needs as the global population continues to grow. Bioenergy crops have received increasing attention as a renewable energy source, but their yields have not been fully realized. In a recent study published in GCB Bioenergy, a team from the University of Illinois assessed the limitations of photosynthetic efficiency of bioenergy species under fluctuating light.

Photosynthesis is a natural process by which plants convert sunlight into energy. Plants have two main photosynthesis, C3 and C4. The two plants differ in that C4 plants have an additional mechanism—phosphoenolpyruvate carboxylase (PEPCase) can be used to transport and supply more CO2 to C3 photosynthesis through a temporarily carbon-fixing carbon concentration mechanism (CCM). Due to this carbon concentration mechanism, C4 plants often have higher water and nitrogen utilization efficiencies.

It is known that photosynthetic efficiency is reduced in fluctuating light, but it is not clear whether C4 plants face reduced efficiency than C3 plants. "We want to know how C3 and C4 plants respond to fluctuating light because one of the limiting factors experienced by farmland crops is the light that changes due to overlapping leaf shading, winds, or passing clouds, and changes in solar angle during the day." Moonsub Lee, a postdoctoral researcher in Illinois, led a research project called "Using ultra-high-yield energy sugarcane to produce renewable oil" (ROGUE). The ROGUE project, led by the University of Illinois Urbana-Champaign, is engineering the two highest-yielding crops in the United States—energycane and miscanthus—to create rich and sustainable oil supply which is used to produce biodiesel, bio-aviation fuel and bio-products. The project was supported by the Office of Biological and Environmental Research and the U.S. Department of Energy.

The investigators of this project aimed to assess the photosynthetic performance of C3 and C4 bioenergy grasses under stable and fluctuating light conditions by examining their leaf gas exchange. They studied 12 different bioenergy grasses, six of which were C3 (for example, rice wheat belonged to C3) and the other six were C4 (e.g., sugarcane, sorghum, corn belonged to C4).

"Overall, C4 species absorb more carbon than C3 species in fluctuating light, and both types absorb about 16% less carbon than expected based on steady state measurements," says Ryan Boyd, a postdoctoral researcher at the Illinois Program to Achieve Enhanced Photosynthetic Efficiency (RIPE). He was also involved in this study of ROGUE. The large variability and natural diversity of C4 responses to fluctuating light provides evidence that photosynthetic efficiency in response to fluctuating light can be used to improve the productivity of C4 bioenergy crops."

While there are not many studies on C3 and C4 species under changing light conditions, the changes found in the results of this study can be used in future plant breeding programs. Researchers believe that it is possible to increase yield and reduce national dependence on fossil fuels for certain characteristics of bioenergy crops as well as other crops. The ultimate goal of this project is to engineer these bioenergy crops so that "high energy density" fuel molecules can be accumulated, and this work is an important step to improve photosynthesis capacity to achieve the goal.

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