Toward a new concept of the evolution of symbiotic nitrogen fixation in the Leguminosae

It is generally believed that only the nodulating species of the Leguminosae fix atmospheric nitrogen; however, anatomical, ecological and taxonomic considerations indicate that non-nodulating legume species may also fix nitrogen. To test whether nitrogen-fixing symbioses in the Leguminosae might extend to the non-nodulating species, a survey of the Leguminosae was conducted: living plants of non-nodulating species were assayed using acetylene reduction. Ethylene evolution, indicating apparent nitrogenase activity, was detected in non-nodulating species representing the major taxonomic groups of Caesalpinioideae as well as in non-nodulating species of the Papilionoideae and Mimosoideae. Non-nodules nitrogen fixation appears to have provided evolutionary precursors for the nodular symbiosis in the Leguminosae.