January 14, 2009
New Study Challenges Theory Behind Methane Production In Plants
New research is challenging the recent finding that plants could be a major source of the atmosphere's methane levels.
A study from 2006 suggested plants could account for almost half of the global production of the greenhouse gas, yet a UK-based team has found that under normal conditions, plants just convey methane from the soil to the air without actually producing it.
The research suggests identifying sources of methane is key for climate control, as the gas is about 20 times more potent than carbon dioxide in terms of its warming effect.
Scientists say methane levels have remained stable for almost a decade, but over the last two years signs show that concentrations have begun to grow again, which according to some observers presages an era of faster-rising temperatures.
The Max Planck Institute for Chemistry in Mainz, Germany, published the research that sparked the debate almost exactly three years ago.
The research team, led by Frank Keppler, found that plants emitted methane from their leaves under normal growing conditions, although the output increased in high sunlight and high temperatures.
The study concluded that plants possessed a hitherto undiscovered biochemical pathway that could generate the gas"”a finding that surprised scientists and other groups who tried to replicate it with mixed results.
Ellen Nisbet, leader of the group that published the latest findings in the Royal Society's Proceedings B, said the old study didn't make sense, and wasn't something that had entered into any of their minds.
"But then we looked at the details of the experiments they'd done, and they were clearly very well done - we just didn't like the conclusions," she said.
Her team grew several different varieties of plant, including maize and rice, in media that contained no organic material, therefore eliminating the chances of methane being formed through decay in soil.
They found that the plants produced no methane at all in closed chambers.
A separate experiment compared the genomes of several plants with those of bacteria that produce methane by biochemical pathways that are well understood.
It concluded that plants could not generate the gas by any known pathway because they do not possess the right genes.
Dr. Nisbet fed basil plants with water containing dissolved methane and later analyzed the air from that chamber, which was found to contain the gas.
The study found, overall, that plants do emit methane during transpiration - the release of water from leaves - but only the methane they have absorbed in water from soil.
"I think this does tell us that the vast majority of methane emitted in normal growth conditions is explained by the absorption of methane in the soil water," said Nisbet.
The analysis showed that plants can begin to decay in stressful conditions, such as high temperatures or high intensities of ultraviolet radiation, which also emits methane - but this is not significant under normal conditions.
The new research does not disprove the idea that a new methane-producing pathway in plants is awaiting discovery, according to Keppler.
"The paper is adding transpiration as a source of methane - that's a nice observation although not entirely new; it's been found in other studies that rice plants act as tubes to conduct methane to the air," he said.
He believes his team clearly showed in previous studies that emissions came from the plant itself and they are now actively looking for that elusive new biochemical pathway.
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