ENHANCING CROP LEGUME N-2 FIXATION THROUGH SELECTION AND BREEDING

Legume N-2 fixation is variable, but nonetheless is a valuable process in world agriculture. There is great potential to increase the contribution by the crop legumes to the world's supply of soil N. This will be achieved by (i) increasing the area of legumes sown by farmers; (ii) improved management of the crops in order that the major determinants of productivity, e.g. land area, water availability, are converted to harvested product with maximum efficiency; and (iii) genetic modification of the commonly-grown species to ensure high dependence of the legume crop on N-2 fixation at all levels of productivity. Currently-used methods for measuring N-2 fixation and for assessing heritability and repeatability of N-2 fixation in breeding and selection programs are reviewed. Results from research programs to define genetic variation in N-2 fixation and to enhance N-2 fixation through selection and breeding are presented with particular emphasis on common bean (Phaseolus vulgaris) and soybean (Glycine max).