They produce about 10 to 30 percent of the annual methane found in the atmosphere, according to researchers at the Max-Planck Institute for Nuclear Physics in Heidelberg, Germany.

The scientists measured the amount of methane released by plants in controlled experiments. They found it increases with rising temperatures and exposure to sunlight.

"Significant methane emissions from both intact plants and detached leaves were observed ... in the laboratory and in the field," Dr Frank Keppler and his team said in a report in the journal Nature.

Methane, which is produced by city rubbish dumps, coal mining, flatulent animals, rice cultivation and peat bogs, is one of the most potent greenhouse gases in terms of its ability to trap heat.

Concentrations of the gas in the atmosphere have almost tripled in the last 150 years. About 600 million tonnes worldwide are produced annually.

The scientists said their finding is important for understanding the link between global warming and a rise in greenhouse gases.

It could also have implications for the Kyoto Protocol, which calls for developed countries to cut their emissions of greenhouse gases by 5.2 percent below 1990 levels by 2008-12.

Keppler and his colleagues discovered that living plants emit 10 to 100 times more methane than dead plants.

Scientists had previously thought that plants could only emit methane in the absence of oxygen.

David Lowe, of the National Institute of Water and Atmospheric Research in New Zealand, said the findings are startling and controversial.

"Keppler and colleagues' finding helps to account for observations from space of incredibly large plumes of methane above tropical forests," he said in a commentary on the research.

But the study also poses questions, such as how such a potentially large source of methane could have been overlooked and how plants produced it.