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Environmental and Stress Physiology

Mechanism of Methane Transport from the Rhizosphere to the Atmosphere through Rice Plants ¹

Division of Agrometeorology, National Institute of Agro-Environmental Sciences, P. O. Box 2, Kannondai, Tsukuba, Ibaraki 305, Japan, Department of Biology, Faculty of Science, Tokyo Metropolitan University, Setagaya-ku, Tokyo 158, Japan, Tokyo Metropolitan Research Institute for Environmental Protection, Koto-ku, Tokyo 136, Japan

To clarify the mechanisms of methane transport from the rhizosphere into the atmosphere through rice plants (Oryza sativa L.), the methane emission rate was measured from a shoot whose roots had been kept in a culture solution with a high methane concentration or exposed to methane gas in the gas phase by using a cylindrical chamber. No clear correlation was observed between change in the transpiration rate and that in the methane emission rate. Methane was mostly released from the culm, which is an aggregation of leaf sheaths, but not from the leaf blade. Micropores which are different from stomata were newly found at the abaxial epidermis of the leaf sheath by scanning electron microscopy. The measured methane emission rate was much higher than the calculated methane emission rate that would result from transpiration and the methane concentration in the culture solution. Rice roots could absorb methane gas in the gas phase without water uptake. These results suggest that methane dissolved in the soil water surrounding the roots diffuses into the cell-wall water of the root cells, gasifies in the root cortex, and then is mostly released through the micropores in the leaf sheaths.

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