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 N2 fixation. A comparative study of growth and rates of N2 fixation over 21 days was conducted with the parental species, F1 hybrids and backcross hybrids, in flowing solution culture, without a supply of mineral N to the plants. T. nigrescens was unable to fix N2 in association with the strains of Rhizobium leguminosarum biovar. trifolii selected for inoculation. Rates of N2 fixation per plant increased in the order T. nigrescens < F1 hybrid < T. repens < backcross 1. Specific rates of N2 fixation (days 0–21) increased in the order T. nigrescens < F1 hybrid < backcross 1 < T. repens. Dry matter production and nodule biomass per plant increased at a higher rate in backcross 1 hybrids than in T. repens. The results suggest that the potential for N2 fixation by backcross 1 hybrids is at least as great as that by T. repens