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Abstract. A fully mechanistic model based on diffusion equations for gas transport in a flooded rice system is presented. The model has transport descriptions for various compartments in the water-saturated soil and within the plant. Plant parameters were estimated from published data and experiments independent of the validation experiment. An independent experiment is described in which the diffusion coefficient of sulfurhexafluoride (SF6) in water-saturated soil was determined. The model was validated by experiments in which transport of SF6 through soil and plant was monitored continuously by photoacoustics. The independent default settings could reasonably predict gas release dynamics in the soil-plant system. Calculated transmissivities and concentration gradients at the default settings show that transport within the soil was the most limiting step in this system, which explains why most gases are released via plantmediated transport. The root-shoot interface represents the major resistance for gas transport within the plant. A sensitivity analysis of the model showed that gas transport in such a system is highly sensitive to the estimation of the diffusion coefficient of SF6, which helps to understand diel patterns found for greenhouse gas emissions, and to the root distribution with depth. This can be understood from the calculated transmissivities. The model is less sensitive to changes in the resistance at the root-shoot interface and in the root fraction active in gas exchange. The model thus provides an understanding of limiting steps in gas transport, but quantitative predictions of in situ gas transport rates will be difficult given the plasticity of root distribution.

Rice plants (Oryza sativa L.) are mainly cultivated in flooded paddy fields and are thus dependent on oxygen transport through the plant to maintain aerobic root metabolism. This gas transport is effectuated through the aerenchyma of roots and shoots. However, the efficiency of gas transport through the rootshoot transition zone is disputed and there are indications that the root-shoot transition zone may represent one of the largest resistances for gas transport. Therefore, we present gas conductance measurements of the root-shoot transition of individual rice tillers measured using SF 6 . SF 6 was detected with a highly advanced laser based photoacoustic detection scheme allowing sensitive, high resolution measurements. In conjunction with these measurements, various plant morphological parameters were quantified. These measurements indeed indicate that the conductance at the root-shoot transition may be much smaller than the conductance of root and shoot aerenchyma within the rice plant. Conductance was strongly correlated to tiller transverse area. After elimination of tiller area from the conductance equation, the resulting permeance coefficient was still correlated to tiller area, but negatively and related to the process of radial tiller expansion. In addition, a decrease in the permeance coefficient was also observed for increasing distance from the plant centre. No correlation was found with tiller type or age of the mother tiller. Incorporation of estimates of the conductance of the root-shoot transition zone coupled to plant morphological parameters will allow considerable improvement of understanding and models on gas transport through plants.