Abstract:

Methane (CH₄) is an important greenhouse gas, and is involved in atmospheric chemical reactions. Aquatic and hydric environments are important sources of atmospheric CH₄. Majority of CH₄ are transported and released to atmosphere by emerged herbaceous plants and hygrophytes in aquatic and hydric environments. In recent decades, there has been increasing attention on how plants transport CH₄. During CH₄ transportation processes, several interfaces of CH₄ exchange play important roles. First, the tips of lateral roots are primary locations (hotspots) for CH₄ entering the root systems and regulate the gross CH₄ transportation. Then, the diaphragms in the aerenchyma and the root collar impose great resistances for the overall CH₄ transportation processes. In early studies, it was controversial that whether CH₄ emission from plants to atmosphere was controlled by stomas or micropores (small cracks and holes in aboveground part of plant except the blade). Recent studies have confirmed the dominant role of micropores for CH₄ transportation and emission. The dead and damaged stems are widely considered to have positive effects on CH₄ transportation. Diffusion and convection are the two main transporting mechanisms of CH₄, with the efficiency of convection being generally higher than that of diffusion. Both biological (e.g. biomass and photosynthesis) and environmental (e.g. light, temperature and humidity) factors regulate the CH₄ transportation. Many studies have contributed to understanding the CH₄ transportation processes and mechanisms by emerged herbaceous plants and hygrophytes. However, there are still some questions needing further investigations. Issues of consideration may include the operational efficiency in the critical interfaces of CH₄ exchange, the plant parts that play a decisive role in the entire CH₄ transportation, the underlying roles of diffusion and convection on CH₄ interfaces exchanges and entire long distance transports, the combined and coupling effects and mechanisms of biotic and abiotic factors, and the similarities and differences of CH₄ transporting processes and mechanisms among plant species.

Key words: Methane transportation, aerenchyma, convection, aquatic plants, wetland plants, greenhouse gases