Root barriers to radial O2 loss restrict CH4 entry and plant-mediated emissions

Rice cultivation is a major source of methane (CH4), with most emissions occurring via plant-mediated diffusion through roots and shoots, and root aerenchyma is widely assumed to facilitate CH4 diffusion from soil to atmosphere. However, the root traits governing radial gas exchange remain poorly resolved, limiting our understanding of plant-mediated emissions. We tested the hypothesis that the barrier to radial O2 loss (ROL) also restricts plant-mediated CH4 emissions using a synthetic experimental approach to isolate plant trait controls from confounding soil microbial processes. Sixteen rice genotypes with contrasting root traits, including a mutant with a constitutively weak ROL barrier, were grown in hydroponics, and plant-mediated CH4 emissions, whole-root ROL, root anatomy, and root system morphology were quantified. Emissions varied widely across genotypes and were positively correlated with whole-root ROL. Neither cortical aerenchyma formation nor root system size explained genotypic variation in CH4 emissions. Scaling by root mass and surface area confirmed that root morphology defines potential emission capacity, but that realised emissions are constrained by the ROL barrier. These findings identify the ROL barrier as a bidirectional diffusion constraint restricting both O2 loss and CH4 intrusion, highlighting whole-root ROL as an integrative physiological trait governing plant-mediated CH4 emissions in rice.