Physiologia Plantarum 134: 303–312. 2008 Copyright ª Physiologia Plantarum 2008, ISSN 0031-9317 Methane emission associated with anatomical and morphophysiological characteristics of rice (Oryza sativa) plant Kaushik Das a, * and Kushal Kumar Baruah b a Department of Crop Physiology, Assam Agricultural University, Jorhat, Assam 785 013, India b Department of Environmental Science, Tezpur University, Tezpur, Assam 784 028, India Correspondence *Corresponding author, e-mail: kaushikdas1111@yahoo.com Received 22 April 2008; revised 14 May 2008 doi: 10.1111/j.1399-3054.2008.01137.x Plant-mediated transport is the primary route of methane (CH 4 ) emission from the reduced paddy field to the aboveground atmosphere. Experiments were conducted at North Bank Plain Agro-climatic Zone of Assam, India, during monsoon rice-growing season (July to December 2006) to elucidate the influences of anatomical and morphophysiological characteristics of rice (Oryza sativa L.) cultivars on methane emission from submerged agro- ecosystem. Ten rice cultivars were grown in light-textured loamy soil under rainfed uniform field condition. Among the 10 cultivars, 5 were traditional rice genotypes commonly grown in the agroclimatic zone and the other 5 were improved high-yielding varieties. Wide variation in CH 4 flux was recorded among the rice cultivars, which may be regulated by the difference in anatomical and morphophysiological characteristics of rice plant. Micro- scopic analysis of stem portion showed that high- and medium-CH 4 -emitting cultivars recorded higher size of the medullary cavity. Leaf area and trans- pirational rates were also found to be higher in high-CH 4 -emitting varieties. Scanning electron microscopic analysis revealed higher stomatal frequencies in high-methane-emitting cultivars. Data presented in this study suggest that variation in anatomical and morphophysiological characteristics among differ- ent rice genotypes may influence CH 4 emission from paddy fields. Introduction The global atmospheric concentration of methane (CH 4 ) has increased from a preindustrial value of about 0.715– 1.774 parts per million by volume in 2005 (IPCC 2007). Methane gas present in the atmosphere substantially affects the radiative budget of the earth and has pre- dominant impact on the global warming. Most of the CH 4 in atmosphere is originated from biological processes in anoxic environment, although there are reports that CH 4 can be emitted from plants under aerobic condition (Keppler and Rockmann 2007, Keppler et al. 2006). Sub- merged paddy fields are one of the dominant anthropo- genic sources of methane to the atmosphere, which is estimated as 15% of the global methane emission (IPCC 1994). Flooding of rice fields promotes anaerobic micro- bial fermentation of organic carbon (Aulakh et al. 2000, Hosono and Nouchi 1997, Jimenez and Lal 2006, Schutz et al. 1989), resulted in extensive methanogenesis in rice rhizosphere. Methane generated by methanogenesis in paddy field is released to the atmosphere by plant- mediated transport, molecular diffusion across the water– air interface and ebullition (Wassmann et al. 1996). The transport of methane through the aerenchyma system of the rice plants is known to be the most important of these (Yagi et al. 1996). Cheng et al. (2006) reported that the CH 4 emitted by plant-mediated transport and Abbreviations – E sif , seasonal integrated flux; SEM, scanning electron microscopy; STP, standard temperature and pressure. Physiol. Plant. 134, 2008 303