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 Legum inosae fix atmospheric nitrogen; however, anatomical, ecological and t axonomic considerations indicate that non-nodulating legume species ma y also fix nitrogen. To test whether nitrogen-fixing symbioses in the Leguminosae might extend to the non-nodulating species, a survey of th e Leguminosae was conducted: living plants of non-nodulating species w ere 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 precur sors for the nodular symbiosis in the Leguminosae.