Nitrogen Fixation by Hybrids of White Clover (Trifolium repens L.) and Trifolium nigrescens

Development of hybrids between white clover (Trifolium repens L.) and Trifolium nigrescens provides a novel route for genetically improving the reproductive capacity of white clover, provided the hybrids are agronomically viable, particularly with respect to N ₂ fixation. A comparative study of growth and rates of N ₂ fixation over 21 days was conducted with the parental species, F ₁ hybrids and backcross hybrids, in flowing solution culture, without a supply of mineral N to the plants. T. nigrescens was unable to fix N ₂ in association with the strains of Rhizobium leguminosarum biovar, trifolii selected for inoculation. Rates of N ₂ fixation per plant increased in the order T. nigrescens < F ₁ hybrid < T. repens < backcross L. Specific rates of N ₂ fixation (days 0-21) increased in the order T. nigrescens < F ₁ hybrid < backcross l < T. repens. Dry matter production and nodule biomass per plant increased at a higher rate in backcross l hybrids than in T. repens. The results suggest that the potential for N ₂ fixation by backcross 1 hybrids is at least as great as that by T. repens.