Nitrogen fixation in several grain legume species with contrasting sensitivities to copper nutrition

The nitrogen fixation response to copper nutrition in faba bean, yellow lupin and soybean was studied. Copper nutrition significantly increased the pod yields of all tested grain legumes but faba bean gave the greatest Cu-use efficiency for pod and grain production. The accumulation of dry matter in vegetative parts, nodules, N and leghemoglobin concentration in nodules and nitrogen accumulation in the whole plants were increased by copper supply in faba bean and yellow lupin in contrast with soybean. Cu nutrition significantly increased the Cu concentrations in nodules of all cultivated plants. The differential sensitivity of N₂ fixation in tested grain legume species to copper nutrition could be connected with the level of phenols in nodules and depended on both the host plants and strains of rhizobia, which differ in their ability to produce catechol-like siderophores. Copper requirements by symbiotic N₂ fixation could also depend on the nature of phenols in nodules (presence of o-dihydroxyphenols or number of hydroxyls in molecule).