Wednesday, November 04, 2009

tiny tubers

The Scientist | 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 of the future had produced 80 percent less food. “It came completely unexpectedly because plants normally grow bigger under higher CO2,” says Gleadow, a plant physiologist at Monash University in Melbourne. Her immediate thought went to the millions of people living in the tropics, where cassava is the third largest source of dietary carbohydrates. “If the yield decreases, there’s going to be a lot of hungry people.”

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).