Methane transport capacity of twenty-two rice cultivars from five major Asian rice-growing countries

Of the methane (CH 4) emitted from a rice field during the growing season, 60–90% is emitted through the rice plants. In order to assess differences among cultivars, we determined the methane transport capacity (MTC) of 18 inbred varieties and 4 hybrids at seedling, panicle initiation (PI), flowering and maturity using an automatic measuring system under greenhouse conditions. All cultivars showed a large increase in MTC from seedling to panicle initiation, but differed in MTC in the succeeding growth stages until maturity. Rice cultivars could be divided into three MTC groups: (a) four hybrids showing no change in flowering to maturity; (b) eight inbred varieties (Zhong yu 906, Zhong zuo 93, Pusa Basmati 1, PR 111, CSR 13, Cisadane, Dodokan, IR72 and IR65598 (new plant type, NPT)) showing either no or minor changes in MTC from panicle initiation to flowering; and (c) nine other inbred varieties (Zhong hua 15, PR 108, Pusa 169, Membarano, Basmati 370, KDML 105, CSR 10, Dular and HTA 60) showing significant increase from panicle initiation to flowering. Groups (b) and (c) had large decreases in MTC at maturity. Differences in MTC patterns among groups were attributed to growth parameters (root, shoot, and root+shoot biomass without grains, and tiller numbers) and the development of morphological characteristics of the aerenchyma. The results suggest that in inbred cultivars representing tall, dwarf and NPT groups, an increase in root or aboveground biomass during plant growth until flowering determines the corresponding increase in MTC. Further increase or any change in plant biomass at maturity does not affect MTC. However, in the case of hybrids, a positive relationship of MTC with root, shoot and total plant biomass including grain indicate continuous development of aerenchyma with plant growth resulting in enhanced MTC. In all cultivars, tiller numbers were linearly related to MTC indicating that the number of outlets determines CH 4 transport. The relative MTC of different cultivars for the complete season ranged from 62 to 445% of IR72. These results clearly demonstrate that rice cultivars differ widely in MTC. Therefore, the use of high-yielding cultivars with low MTC (e.g. KDML 105, IR65598 and PR 108) could be an economically feasible, environmentally sound and promising approach to mitigate CH 4 emissions from rice fields.