Rhizobia interact symbiotically with legumes to form root nodules, where by rhizobia fix atmospheric dinitrogen into ammonia in exchange for carbon produced via photosynthesis. The symbiotic interaction is agriculturally important by reducing the need for fertilizer containing nitrogen. The root and stem nodule bacteria commonly include bacteria in the genera Rhizobium, Mesorhizobium, Sinorhizobium (Ensifer), and Bradyrhizobium, although other genera of bacteria have now been shown to form root nodule symbioses with several legume species (Weir, 2012). Different rhizobial strains form different numbers of nodules on specific legume plant varieties (or cultivars), and the nitrogen fixing effectiveness of each rhizobial strain, its ability to fix nitrogen and transfer it to the plant, is also highly variable (Toro, 1996). Some native rhizobia are ineffective at fixing nitrogen yet form a majority of nodules in filed grown plants. This is referred to as the competition for nodulation problem (Triplett and Sadowsky, 1992). Competition studies are not feasible when evaluating a large number of different native strains. However, nodulation speed of individual strains correlates well with overall competiveness and can be used to identify native strains that overcome the competition problem (De Oliveira and Graham, 1990; Bhuvaneswari et al., 1980).
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