Despite often growing in water, the giant reed Arundo donax is not particularly wasteful of water, while being astonishingly efficient at photosynthesis. Stephen Bidouze/Shutterstock

A species of giant reed is unique in the plant kingdom, growing with a speed only matched by plants that use a very different method of photosynthesis. For the first time, we have some answers on how Arundo donax achieves its efficiency. This may offer researchers pointers in making more efficient use of sunlight and water.

There are three pathways, known as CAM, C3, and C4, in which plants use sunlight to combine carbon dioxide and water into the sugar that forms their energy store. Most plants have an efficiency of just 3 to 6 percent, so even small improvements matter a lot. C3 is the simplest and by far the most common form, but C4 plants are more efficient in hot conditions or when water is in short supply. C4 grasses can lose half as much water as C3 grasses grown under the same conditions, and grow much faster.

In this context, A. donax represents a puzzle. The Mediterranean reed is a C3 plant that sucks down carbon dioxide as fast as a C4. In one side-by-side trial, it comfortably outgrew the famously productive C4 elephant grass (Miscanthus x giganteus). A team at the University of Illinois set out to see how it does this. According to senior author Professor Stephen Long, the first step was to confirm, contrary to some speculation, that A. donax really does use C3. "It has all the properties of C3 photosynthesis and none of those of C4," Long said in a statement.

Another theory Long disproved is that A. donax achieves this growth by consuming huge amounts of water. "Giant reed is a deep-rooting plant, so it's able to get plenty of water. But it's not using that water any less efficiently than other C3 plants," Long said.

Part of the answer is that while most plants are adapted to either shade or direct sun, A. donax performs well in both. "The lower leaves are producing a lot of chlorophyll to capture what light is getting to them, and the plant is using that at maximum efficiency," said Long.

The plant manages to transport electrons through its leaves unusually fast and has very high activity of Rubisco, the enzyme used to catalyze photosynthesis in C3 and C4 plants. The work did not, however, explain why other plants have not replicated A. donax's impressive capacities.

Understanding A. donax's success is important both because we might want to use it directly as a carbon neutral fuel, but also in case its attributes prove transferable to other plants. Rice is being genetically engineered to use the C4 pathway for faster growth. Other researchers have sought even more exotic ways of supercharging photosynthesis with genes from blue-green algae. If similar potential exists in C3 plants, it might be easier to transfer to major food crops.

Traditional cross-breeding techniques, however, will probably not be able to spread A. donax's useful genes. The paper notes that its "spread has not involved hybridization with the related species," suggesting this really is a reed apart.

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