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Not guilty. Plants don't appear to produce methane, as had been previously reported.

Euan Nisbet

Methane Emissions? Don't Blame Plants

Plants do not make the powerful greenhouse gas methane, according to new research that contradicts a controversial finding made in 2006. Instead, plants appear to merely be passing gas, so to speak, originally made by soil microbes.

Methane comes from a variety of sources, including gas leaks, forest fires, and, of course, cow burps. Microbes in wetland soil can produce methane anaerobically (without using oxygen), but the idea that it can be produced aerobically (using oxygen) by plants, and on a large scale, is still extremely controversial. In 2006, geochemist Frank Keppler of the Max Planck Institute of Nuclear Physics in Heidelberg, Germany, conducted experiments on dead leaves and in greenhouses and concluded that many kinds of plants--through some mysterious mechanism--contribute to methane production. All told, plants could be to blame for 10% to 45% of the world's methane emissions, Keppler reported (Science, 13 January 2006, p. 159).

"This finding was shocking," recalls Euan Nisbet of Royal Holloway, University of London, in Egham, U.K. If true, both plant biochemistry and global methane budget would need a major reexamination. It could also mean that the human contribution to global warming is less than previously thought.

Nisbet's team set about to investigate Keppler's findings by growing the same plants, including celery (Apium graveolens) and a type of rice (Oryza sativa), in the absence of external sources of the greenhouse gas. The group found no trace of methane, suggesting that the plants alone cannot make the gas. In a separate experiment, the team placed the plants in water containing dissolved methane. Sure enough, the roots drew up the methane-soaked water and the leaves then pushed out the gas and water vapor--a process known as transpiration.

The researchers also tried to find a chemical pathway by which the plants could make methane aerobically. They came up empty: None of the plants' genes codes for enzymes similar to those made in methane-producing microbes. "This showed that the plants were not guilty," says co-author Christopher Howe of the University of Cambridge in the U.K. The findings are published online today in the Proceedings of the Royal Society B.

Keppler, now at the Max Planck Institute for Chemistry in Mainz, agrees with some of the team's conclusions, saying that transpiration does play a role in plant emissions of methane. But he still holds firm that methane can be produced in plants via a new, unidentified biochemical pathway. Nisbet is skeptical: "We're not saying it is not there, but we certainly couldn't find it."