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. 

To examine nodulation and effectiveness of rhizobia in terms of promoting plant growth, rhizobia were isolated from field populations of the invasive legume Lupinus polyphyllus around the Turku area in southwestern Finland in April 2021. Two-month inoculation experiments with six different bradyrhizobial strains were then conducted in greenhouse and common-garden conditions in June-July 2022 to quantify plant growth. Each experiment contained 288 plants in the beginning of the experiments that were inoculated with either 1 ml of the growth medium without rhizobia (the control treatment, C) or one of the six rhizobial strains (R1-R6). In a positive control, plants were inoculated with 1 ml of the growth medium without rhizobia and were then fertilised twice during the experiment with a commercial NPK fertilizer (the nitrogen-addition treatment, N). 

The following variables were recorded from the plants: plant initial height one week after planting, plant height (three times during the experiment; three, five, and seven weeks after planting), number of leaflets per leaf, the length of the longest leaf, number of nodules, colour of dissected nodules (in the greenhouse experiment only), dry biomass for shoot, root and nodules, survival.

In the greenhouse data, nine and five individuals that formed nodules in the control (C) and nitrogen-addition (N) treatments, respectively, were omitted. In the common garden data, three individuals that formed nodules in the control treatment were omitted.