Rhizobial inoculation experiments for the invasive legume Lupinus polyphyllus

Background and Aims: For invasive plant species that associate with
mutualistic symbionts, partner quality can be critical to their invasion
success. This might be particularly true for legumes that host
nitrogen-fixing bacteria (rhizobia). Here, we examined the relative
effectiveness of rhizobial strains on the invasive legume Lupinus
polyphyllus.  Methods: We isolated rhizobia from field
populations of L. polyphyllus and conducted inoculation experiments in
which we quantified plant growth in greenhouse and common-garden
conditions. Results: Differences in nodulation and effectiveness in terms
of increasing plant growth among rhizobial strains of the genus
Bradyrhizobium were more pronounced in the greenhouse than in the common
garden. All six rhizobial strains nodulated the host plant in greenhouse
conditions, but one failed to nodulate in the common garden. Under
greenhouse conditions, five rhizobial strains increased plant biomass by
66–110%, while one provided negligible benefits compared to control plants
without rhizobia, suggesting that rhizobial identity might be critical to
the invader’s performance. However, the common-garden experiment revealed
no differences in the effectiveness of rhizobial strains in terms of plant
biomass, number of leaflets per leaf, height, root:shoot ratio, or
survival. Moreover, the performance of rhizobia-inoculated plants in the
common garden did not differ from plants without rhizobia, which may call
into question the fitness benefits of rhizobia to field populations of
this species.  Conclusions: The discrepancies observed
between the two environments highlight the importance of considering
field-realistic growing conditions and multiple plant traits when
assessing the potential growth benefits of symbiotic partners to host
plants.