Role of plant physiology varietal differences

Energy supply to microorganisms and anoxic conditions controlling methanogenesis may also be influenced by the growing rice plant. Some energy is supplied to CH4-generating organisms in the form of dissolved organic carbon released by the growing plant roots into the surrounding soil. Influenced by plant physiology, release rates seem to be variety-dependent (Sigren et al, 1997; Inubushi et al, 2003; Lou et al, 2008). Varieties may also differ in the degree to which they promote oxygen diffusion into the root environment (Satpathy et al, 1998). Greater rates of oxygen diffusion from roots into the soil could stimulate CH4 oxidation and inhibit CH4 production by increasing the oxygen pressure at the soil-root interface. Among ten high-yielding varieties tested in India, fourfold differences in seasonal CH4 emissions were observed between varieties producing similar grain yields, while no correlation between emissions and yield was observed (Satpathy et al, 1998) (Figure 8.4).

Figure 8.4 Variation in relationship between yield and CH4 emissions, for ten high-yielding Indian rice varieties

Source: Based on data from Satpathy et al (1998)

In the US, Huang et al (1997) found a positive correlation between grain yield and CH4 emissions among ten varieties tested under the same conditions. However, a high emission was not always found to accompany high yield. Also in the US, a positive correlation was found between plant height and CH4 emission (Ding et al, 1999), and semi-dwarf varieties were found to emit 36 per cent less CH4 than tall ones (Lindau et al, 1995). In the largest study of this kind, involving a total of 29 varieties, results were inconsistent (Wassmann et al, 2002). Within a given season differences were small; compared during nine seasons under varying environmental conditions, most were insignificant. Wassmann et al (2002) concluded that the variety-specific differences were small compared to other factors, varied between seasons and were too elusive for solid classification of varieties with respect to their CH4 mitigation potential. However, since there are hundreds of high-yielding rice varieties in use and new ones are constantly being bred, this is certainly not the last word on this issue, and efforts to identify mechanisms responsible for variety-dependent CH4 emissions are continuing.

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