Methane Transport Capacity of Rice Plants. I. Influence of Methane Concentration and Growth Stage Analyzed with an Automated Measuring System

A major portion (60–90%) of the methane (CH₄) emitted from rice fields to the atmosphere is transported through the aerenchyma of the rice plants. However, a rapid and accurate method to study the CH₄ transport capacity (MTC) of rice plants is not available. We developed a gas sampling and analytical device based on a closed two-compartment chamber technique and analyzed the enrichment of the CH₄ mixing ratio inside the shoot compartment of cylindrical cuvettes enclosing individual rice plants under ambient conditions. The computer-controlled analytical system consists of a gas chromatograph (GC) and a pressure-controlled autosampler for eight cuvettes (seven for plants and one for CH₄-calibration gas). The system automates closure and opening of plant cuvettes using pneumatic pressure, air sample collection and injection into the GC, and CH₄ analysis. It minimizes sources of error during air sampling by continuously mixing headspace air of each cuvette, maintaining pressure and composition of the headspace inside the cuvettes, purging the dead volumes between the sampler induction tube and GC, and running a reference CH₄-calibration gas sample in each cycle. Tests showed that the automated system is a useful tool for accurate sampling of headspace air of cylindrical cuvettes enclosing individual rice plants and enables rapid and accurate fully automated analysis of CH₄ in the headspace air samples. A linear relationship was obtained between CH₄ transported by rice plants of two cultivars (IR72, a high-yielding dwarf, and Dular, a traditional tall cultivar) and concentration of CH₄ up to 7,500 ppm used for purging the nutrient culture solution surrounding the roots in the root compartment of the chamber. Further increase in CH₄ emission by shoots was not observed at 10,000 ppm CH₄ concentration in the root compartment of the chamber. The MTC of IR72 was measured at six development stages; it was lowest at seedling stage, increasing gradually until panicle initiation. There was no further change at flowering, but a marked decrease at maturity was noted. These results suggest that the plants have 45–246% greater potential to transport CH₄ than the highest CH₄ emission rates reported under field conditions, and plants would not emit CH₄ at early growth and at a reduced rate close to ripening.