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.