Nitrogen fixation by hybrids of white clover (Trifolium repens L.) and Trifolium nigrescens

Development of hybrids between white clover (Trifolium repens L.) acid Trif olium nigrescens provides a novel route for genetically improving the repro ductive capacity of white clover, provided the hybrids are agronomically vi able, particularly with respect to N-2 fixation. A comparative study of gro wth 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, wi thout 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. trif olii selected for inoculation. Rates of Nz fixation per plant increased in the order T. nigrescens < F-1 hybrid < T. repens < backcross 1. Specific ra tes of N-2 fixation (days 0-21) increased in the order T. nigrescens < F-1 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. repe ns. The results suggest that the potential for N-2 fixation by backcross 1 hybrids is at least as great as that by T. repens.