The physiology of nitrogen fixation in tropical grain legumes

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Abstract
In the tropics, improving protein production depends on increasing agricultural productivity while maintaining low production costs. A major factor limiting productivity is nitrogen fertilizer and, in this context, grain legumes are a foremost alternative because of their high protein content and ability to obtain fixed nitrogen via biological fixation. Nitrogen fixation in the legume/Rhizobium symbiotic system is the result of a complex integration of physiological functions between the host plant and the endophyte. It depends on an exchange of specific metabolities: the bacteroids have an obligatory requirement for carbon substrates as their source of energy and carbon skeletons for nitrogen fixation and assimilation, and the plant receives the nitrogenous products exported from nodules which are required for protein synthesis. Maximizing biological nitrogen fixation to provide nitrogen for high and reliable yields of grain legumes clearly requires an understanding of the functioning of the symbiotic system, the integrated carbon and nitrogen economies, the processes limiting nodule activity and efficiency, as well as the environmental factors potentially detrimental to the plant/bacteria association.