ABSTRACT
The ability of legumes to enrich the soil for subsequent crops was known in Graeco-Roman times, though the mechanism of nitrogen fixation was not discovered until the late nineteenth century and the bacteria – collectively known as rhizobia – responsible for the legume plant-bacterial symbiosis were not identified until the mid-twentieth century. Atmospheric di-nitrogen (N2) is fixed by bacteria of the genera Rhizobium and Bradyrhizobium, which infect the root hairs of the legume families and thus induce the formation of benign plant galls (nodules) and live symbiotically with the host plant. Rhizobial species are specific to individual host legumes or groups of legumes, but among legume species there are great variations in specificity of interaction with rhizobia. For example, the alfalfa and melilotus host group requires Sinorhizobium meliloti, while the clover group, which includes white, red and subterranean clovers, requires Rhizobium leguminosarum bv. trifolii, and Mesorhizobium loti colonizes birdsfoot trefoil roots (Amarger, 2001).
