ORIGINAL PAPER Association between contrasting methane emissions of two rice (Oryza sativa L.) cultivars from the irrigated agroecosystem of northeast India and their growth and photosynthetic characteristics Kaushik Das Æ K. K. Baruah Received: 3 December 2007 / Revised: 12 February 2008 / Accepted: 22 February 2008 / Published online: 2 April 2008 Ó Franciszek Go ´rski Institute of Plant Physiology, Polish Academy of Sciences, Krako ´w 2008 Abstract Over two consecutive years in the North Bank Plain Zone of Assam, India, during the spring growing season (February–June) of- 2006 and 2007 we examined effects of morpho-physiological characteristics of rice (Oryza sativa L.) plants in relation to methane (CH 4 ) emission from paddy fields. Traditional cultivar ‘‘Agni’’ and modern improved cultivar ‘‘Ranjit’’ were grown in light textured loamy soil under irrigation. A higher seasonal integrated methane flux (E sif ) was recorded from ‘‘Agni’’ compared to ‘‘Ranjit’’. Both cultivars exhibited an emission peak during active vegetative growth and a second peak at panicle initiation. Leaf and tiller number, leaf area, length, and volume of root were greater in ‘‘Agni’’, but grain yield and yield-related parameters such as increased photosyn- thate partitioning to panicles at the expense of roots were greater in ‘‘Ranjit’’. ‘‘Ranjit’’ also photosynthesed faster than ‘‘Agni’’ during panicle development but slower than ‘‘Agni’’ at tillering. In both the years, a higher soil organic carbon content was recorded in plots of ‘‘Agni’’. Our results suggest that in ‘‘Agni’’ enhanced diversion of photosynthate to roots resulted in more substrate being available to methanogenic bacteria in the rhizosphere. Additionally, the more extensive vegetative growth of this cultivar may enhance methane transport from the soil to the above- ground atmosphere. Keywords Growth Á Methane Á Organic carbon Á Photosynthesis Á Rice Introduction The global atmospheric concentration of methane (CH 4 ) has increased from a pre-industrial value of about 715– 1,732 ppb by the early 1990s and to 1774 ppb by 2005 (IPCC 2007). Methane is an important greenhouse gas, and the atmospheric increases account for about 15% of current global warming (Batjes and Bridges 1992). Most methane in the atmosphere originates from anaerobic biology. Although, annual increases in atmospheric CH 4 have slo- wed recently, possibly because of decreasing anthropogenic emissions from fossil fuel, this is thought to be only a temporary pause (Bousquet et al. 2006). Most rice (Oryza sativa L.) is cultivated in submerged anoxic soils. These have been identified as a major source of anthropogenic methane contributing about 10– 15% to global CH 4 emissions (Neue 1993). Rice grow- ing in India can be broadly categorized into rainfed and irrigated farming representing about 52 and 48% of the total rice area, respectively (Babu et al. 2006). Methane production in rice fields such as these is the result of interactions of soil processes involving plants and microbes (Verburg et al. 2006). Flooding the soil pro- motes anaerobic degradation of photosynthetic carbon supplied by rice plants resulting in methane production whenever redox potentials become sufficiently negative. The major sources of substrate for methanogenic bac- teria are root exudates and decaying plant remains (Jimenez and Lal 2006). Therefore, the rate of produc- tion and emission of methane is thought to depend on morpho-physiological parameters such as growth and Communicated by M. B. Jackson. K. Das (&) Á K. K. Baruah Department of Environmental Science, Tezpur University, Tezpur, Assam 784 028, India e-mail: kaushikdas1111@yahoo.com K. Das Department of Crop Physiology, Assam Agricultural University, Jorhat, Assam 785 013, India 123 Acta Physiol Plant (2008) 30:569–578 DOI 10.1007/s11738-008-0156-4