Mechanism of Methane Transport by Rice Plants
To clarify the mechanism of CH4 transport through rice plants, CH4 formation in flooded soil and emission through rice plants were examined by pot culture (soil and hydroponic culture) experiments. Methane concentration in surface water above soil was extremely low as compared to that in soil water. Methane concentration in vegetated soil was about one-third of that in unvegetated pots. The emission rate was 20 times higher in vegetated than in unvegetated pots. In addition, there was a linear relationship between CH4 emission rate from rice plants and CH4 concentration in the culture solution with a high CH4 concentration. Methane transport capacity of rice plants depended mainly on plant size. These results indicate that rice plants have a large capacity for CH4 transport and that rice plants play a primary role for CH4 flux from paddy fields.
Methane was not emitted from stomata. Air bubbles or droplets of cupric sulfate were observed on leaf sheaths at the lower leaf position by injection of air or cupric sulfate solution into the medullary cavity of rice plants. Micropores which are different from stomata were observed at the abaxial epidermis of leaf sheath by scanning electron microscopy. These results indicate that the micro-pores are the main site of CH4 release from rice plants. The present results indicate that the seasonal peak of CH4 flux from paddy fields results from a combination of CH4 concentration in flooded soil and rice plant growth since CH4 formed in the rhizosphere was transported to the atmosphere through the rice body by a physical process.
KeywordsSoil Water Rice Plant Emission Rate Paddy Field Rice Straw
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