Mechanism of CH4 Transport from Rice Paddies through Rice Plants
Abstract
This study clarified that methane (dissolved by rice plants) generated from paddy soil is
absorbed from the root and released into the atmosphere through the micropores in the
leaf sheath of rice plants.
Research Institute
National Institute of Agro-Environmental Sciences
Background and Purpose
In recent years, atmospheric methane (CH4) has increased approximately 1% each
year. Because CH4 is one of the so-called greenhouse gases, like carbon dioxide (CO2)
and chlorofluorocarbons, the increase in atmospheric methane may cause global
warming and environment problems. It is known that methanogenic bacteria in flooded
soil and anaerobic bacteria in the intestines of animals can produce methane. Rice
paddy fields are known to be a major source of methane, but how the methane is
transported from paddy soil into the atmosphere still remains uncertain. Therefore, the
purpose of our research is to clarify the mechanism of CH4 transport from rice paddies.
Contents and Characteristics
1.
Methane concentration in the medullary cavities of rice plants was 500 to 5,000
ppm and was higher in the lower medullary cavity than that in the upper one.
2.
The roots of a rice plant were soaked in culture solution with high CH4
concentration (prepared by bubbling 40% methane into the culture solution), the
shoots were enclosed in a cylindrical chamber (3 cm in diameter and 50 cm in
length), and ambient air was introduced from the bottom of chamber. A long
stainless steel tube (1 mm in diameter and 60 cm in length) was inserted into the
top of the chamber. Air was sampled at each height of the chamber. Methane was
mostly released from the culms, which was an aggregation of leaf sheaths, but not
from the leaf blade (Figure 1).
3.
When the rice stem was cut off near the root, the upper part with leaves was soaked
in water and then air was injected through a syringe, many air bubbles were
observed from both abaxial epidermis of the leaf sheath and bottom of the leaf nodal
plate.
4.
An scanning electron microscope was used to examine the portions where air
bubbles appeared. Micropores (4μm in diameter) which are different from stomata
１on leaf veins were arranged regularly (Figure 2).
5.
When the rice root was placed in gas phase and exposed to 10 to 100% CH4 gas, CH4
was released from the aerial part of the rice plant in the same way as in the liquid
phase.
6.
Methane from rice paddies diffuses into the cell-wall water of the root cells and root
cortex. Then it quickly gasifies into the root cortex, and is transported by diffusion
to the shoots via the lysigenous intercellular spaces and aerenchyma. Eventually, it
was gradually released through the micropores in the epidermis of the leaf sheaths
of rice roots and stomata in the leaf blade.
Application and Notes
The research clarified the role of rice plants in CH4 transport and contributed the
basic data to development of CH4 restraint technology from rice paddies.
Figure 1. CH4 concentration at each position of shoot in the cylindrical chamber
Figure 2. Scanning electron micrograph of micropores (↓) which are thought to be the
site of CH4 release in the epidermis of the lower portion of the leaf sheath. Scale bar,
13.6um, magnification, 2,000