DOI: 10.5061/dryad.2jm63xsv1
title: Rhizobial inoculation experiments for the invasive legume Lupinus polyphyllus

description: 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. 
description: To examine nodulation and
 effectiveness of rhizobia in terms of promoting plant growth, rhizobia
 were isolated from field populations of the invasive legume
 <em>Lupinus</em> <em>polyphyllus</em>
 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.