Selecting improved Lotus nodulating rhizobia to expedite the development of new forage species

AIMS: In the past decades the increasing focus by Australian pasture development programs on the genus Lotus has seen the evaluation of many species previously untested in Australia. In field trials, nodulation failure was commonplace. This work was undertaken to select effective symbionts for Lotus to ensure further agronomic evaluation of the genus was not compromised. The symbiotic needs of Lotus ornithopodioides were a particular focus of the studies. METHODS: Glasshouse experiments were undertaken to evaluate symbiotic relationships between 15 Lotus spp and 23 strains of nodulating Mesorhizobium loti. This was followed by evaluation of elite rhizobial strains for their ability to persist and form nodules under field conditions. RESULTS: Complex symbiotic interactions were recorded between strains of lotus rhizobia and the different species of Lotus. Notably, the rhizobia that are currently provided commercially in Australia for the inoculation of Lotus corniculatus (strain SU343) and Lotus uliginosus (strain CC829) did not form effective symbioses with the promising species L. ornithopodioides and L. maroccanus. No strain we evaluated was compatible with all the Lotus species, however several strains with a broad host range were identified. WSM1293 and WSM1348 were the most effective strains on L. ornithopodioides and L. peregrinus.These strains were also moderately effective on L. corniculatus (79 and 52% of SU343), less effective on L. maroccanus (26 and 49% of SRDI110) but were ineffective on L. uliginosus. The latter species overall had very specific rhizobial needs. Both WSM1293 and WSM1348 produced adequate levels of nodulation when inoculated on L. ornithopodioides, over two seasons at three field sites. CONCLUSIONS: Effective and persistent strains are now available that should allow the un-compromised evaluation of many of the contemporary Lotus species in the field. Selecting a strain for use in commercial inoculants will be more problematic, given the very large host-strain interactions for nitrogen fixation. Here, the balance of Lotus species which are adopted by farmers will have a strong bearing on which rhizobial strains are progressed to commerce.