Breeding for enhanced nitrogen fixation in crop legumes

Legume dinitrogen (N-2) fixation is a variable, but nonetheless valuable process in world agriculture, contributing almost 20% of the nitrogen (N) needed for world grain and oilseed production. For all crop legumes, there is great potential to increase the percentage of legume N derived from N-2 fixation as well to enhance the total N-2 fixed through improved management and generic modification of the plant. Strategies and operational frameworks for conducting selection and breeding programs to enhance N-2 fixation, and methods for measuring N-2 fixation that have been used in such programs, are reviewed. Results from research programs to define genetic variation in N-2 fixation of soybean (Glycine max) and to enhance N-2 fixation through breeding are presented, with emphasis on symbiotic tolerance to nitrate, mutagenesis-induced supernodulation and promiscuous and selective nodulation. Whilst genetically based variation in N-2 fixation traits has now been demonstrated for soybean and other legume species, incorporating such variation into cultivars has had little success. Future programs may benefit from increased integration into mainstream legume breeding programs that are focussed on a broad range of traits, and in which all material is evaluated in low N soils to provide the most appropriate selection pressure for highly effective nodulation and N-2 fixation. (C) 2000 Elsevier Science B.V. All rights reserved.