By Richard Black
Environment correspondent, BBC News website
Research suggests plants release methane that they have absorbed
The recent finding that plants could be a major source of the atmosphere's methane is challenged by new research.
A 2006 study suggested plants could account for almost half of the global production of the greenhouse gas.
But a UK-based team now reports that under normal conditions, plants just convey methane from the soil to the air without actually producing it.
Writing in a Royal Society journal, they suggest identifying sources of methane is key for climate control.
The gas is about 20 times more potent than carbon dioxide in terms of its warming effect.
After remaining stable for almost a decade, there have been signs in the last two years that concentrations have begun to grow again, which according to some observers presages an era of faster-rising temperatures.
The research that sparked the debate was published almost exactly three years ago by a group from the Max Planck Institute for Chemistry in Mainz, Germany.
Frank Keppler's team found that plants emitted methane from their leaves under normal growing conditions, although the output increased in high sunlight and high temperatures.
They suggested that plants possessed a hitherto undiscovered biochemical pathway that could generate the gas.
The finding surprised scientists, and other groups tried to replicate it - with mixed results.
"It just didn't make sense, and wasn't something that had entered into any of our minds," said Ellen Nisbet, leader of the group that has just published the latest findings in the Royal Society's Proceedings B.
"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 told BBC News.
Part of her team's work involved growing several different varieties of plant, including maize and rice, in media that contained no organic material, so eliminating the chances of methane being formed through decay in soil.
They found during these experiments, conducted in closed chambers, that the plants produced no methane at all.
Reforming the way rice is farmed could reduce methane emissions
In another experiment, they compared the genomes of several plants with those of bacteria that produce methane by biochemical pathways that are well understood.
Plants, they concluded, could not generate the gas by any known pathway because they do not possess the right genes.
In a third strand of investigation, Dr Nisbet fed basil plants with water containing dissolved methane.
Later, the air from that chamber was analysed and found to contain the gas. The team concludes 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 Dr Nisbet, who conducted the research at the University of Cambridge but who now works at the University of South Australia in Adelaide.
In stressful conditions, such as high temperatures or high intensities of ultraviolet radiation, plants can begin to decay, which also emits methane - but this is not significant under normal conditions, according to this analysis.
Hand in the till
Dr Keppler suggested the new research does not disprove his idea that a new methane-producing pathway in plants is awaiting discovery.
"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 told BBC News.
"But we clearly showed in previous studies that emissions came from the plant itself."
His research team is now actively looking for that elusive new biochemical pathway.
So the issue is clearly not completely settled yet.
There are hints that atmospheric methane levels are rising again
In the meantime, Ellen Nisbet suggests there is potential to reduce methane emissions from rice paddies by using farming techniques that ensure fields are always oxygenated, which inhibits methane-producing decay.
In recent years there has been a lot of interest in "no-till" or "minimum-till" methods that disturb soil less than conventional farming, helping the earth to store carbon.
Some studies suggest the same technique might also restrain methane release.