Nitrogen fixation in hybrids of white clover (Trifolium repens L.) and Caucasian clover (Trifolium ambiguum M. Bieb)

Hybrids between white clover (Trifolium repens L.) and Caucasian clover (or Kura clover, Trifolium ambiguum M. Bieb) are a potential route for the improvement of drought tolerance and persistence in white clover. However, to be agronomically viable they must show no significant reduction in their potential for nitrogen fixation relative to white clover. A comparative study of growth rate and nitrogen fixation was carried out in flowing solution culture without a supply of mineral nitrogen to the plants. The two parental species and two generations of backcross hybrids, with white clover as the recurrent parent, were assessed. The growth rate and N content of T. ambiguum were significantly lower than those of the other lines. However, dry matter production, nodule biomass per plant and rates of fixation were similar in second-generation backcross plants and white clover. The results suggest that the agronomic potential of this novel germplasm is not compromised by limitations with respect to nitrogen fixation.