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Tiny tubers

By Daniel Grushkin Tiny tubers A cassava grown under CO2 levels projected for the next century (right) yielded 80% less food than a tuber grown under current conditions. Courtesy of Plant Biol, published online August 6, 2009 When Ros Gleadow opened the airlock to the greenhouse at The Australian National University, she stepped into the atmosphere of the future. The air was thick with carbon dioxide—700 parts per million, to be precise—

Daniel Grushkin

Tiny tubers

A cassava grown under CO2 levels projected for the next century (right) yielded 80% less food than a tuber grown under current conditions.
Courtesy of Plant Biol, published online August 6, 2009

When Ros Gleadow opened the airlock to the greenhouse at The Australian National University, she stepped into the atmosphere of the future. The air was thick with carbon dioxide—700 parts per million, to be precise—which matches the concentration predicted 90 years from now. While evaluating the responses of crops to the altered atmosphere in the summer of 2008, she found that the cotton, sorghum, soybean and cassava plants she’d planted 9 months earlier grew higher, a little woodier, and with more stems and smaller leaves than normal—all of which she’d expected. But when she dug the cassavas out of their pots, the tubers, which usually grow as large as yams, looked like stunted fingers.

Her cassavas...

That wasn’t the only problem. The cassava plants themselves had become poisonous. Like 60 percent of all our staple crops, cassava produces chemicals called cyanogenic glycosides to deter grazing animals, which, when chewed, release cyanide gas. In small quantities, the cyanide tastes like bitter cherries, enough to ward off animals. But the high-CO2 cassavas produced three times the cyanide of today’s plant. (The poison largely shows up in the leaves, which most people avoid, although some in African countries eat the leaves as a protein supplement.) Gleadow hypothesizes that her cassavas may have poisoned themselves, meaning the extra cyanide shrank the tubers (Plant Biology, published online August 6, 2009).

Until recently, modelers saw CO2’s effect on plant life as the silver lining of climate change. They thought increases in the gas would act as fertilizer, making crops grow bigger and more lush. After all, CO2 is one of the main components of photosynthesis. In the late 1980s, experimenters projected as much as 30 percent increases by 2050 in yield for staples like wheat and soy. But recent experiments under open-air conditions showed half that rate of growth (Science, 312: 1918–21, 2006).

The tubers, which usually grow as large as yams, looked like stunted fingers.

The effects of higher CO2 tend to be more nuanced than first projected and, in cases like cassava, species-specific. The plants did devote the extra energy from higher CO2 to their carbon skeletons, growing taller, thicker stems and branches as well as fewer leaves. In cases such as cassava’s, though, the plant devoted part of the added bounty to developing defenses, i.e., producing cyanide. “You’re affecting plants at the heart of their metabolism, so a lot of things change about them, including their chemistry,” says Daniel Taub, a biologist at Southwestern University in Georgetown, Texas.

At the root of the change is a series of microscopic stomata, lip-shaped pores on leaves that plants use to absorb CO2. Under higher levels of CO2, stomata partially shut in C3 plants (plants that convert CO2 into three carbon molecules) like rice, wheat and cassava. Stomata have a secondary function: they emit vapor so that plants can siphon water and nutrients from the soil into their branches, a process called transpiration. Because the stomata contract, plants transpire less and use less water, thus drawing fewer nutrients from the soil. As a result, C3 crops exposed to more CO2 show deficits by up to 15 percent in calcium, magnesium, phosphorus, and—most important—protein (Glob Chang Biol 14: 565–75, 2008).

In November, Gleadow heads off to Mozambique on a grant from the Australian government to examine the effects of elevated CO2 on yams and taro. She and other scientists are also investigating which crops might fare best in the air of the next century, and are breeding them. “I think I know what I’m going to be doing for the next 15 years,” Gleadow says.

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