Allelopathic effects of Alliaria petiolata on rhizobia and its implications for native legume performance

Background/Question/Methods

Invasive species threaten native plant communities, reducing biodiversity and changing community composition. Alliaria petiolata (garlic mustard), an invasive plant species in North America, has shown to outcompete native species indirectly through its use of novel allelochemicals that reduce performance of mycorrhizal fungi mutualists, upon which many native plant species rely. It is possible that these allelopathic effects extend to other mutualists. Here, we test the effects of A. petiolata’s allelochemicals on rhizobia, an N-fixing bacteria and important symbiont for native legumes, and on the benefits rhizobia confer to their hosts. First, we conducted a laboratory experiment to test whether A. petoilata’s allelochemicals have direct negative effects on rhizobia growth by growing five rhizobium strains in liquid media with three concentrations of A. petiolata extract. Then, we performed a greenhouse experiment to test whether A. petiolata allelochemicals influence legume-rhizobium interactions and the fitness benefits the native legume, Amphicarpaea bracteata, receives from rhizobia. The experimental design was 2x4 factorial, treating A. bracteata seedlings with rhizobia [presence and absence] and plant tissue additions [fresh A. petiolata, Hesperis matronalis (non-invasive mustard), Trifolium pratense (non-invasive legume), and control]. Six weeks later, we measured above- and below- ground biomass of A. bracteata and counted nodule number. 

Results/Conclusions

We found A. petiolata extracts reduced maximum grow rate (µmax) for all rhizobium strains tested (F = 7.24; P = 0.002), and we found no difference among strains (F = 1.32; P = 0.28). This may suggest that A. petiolata’s allelochemicals have direct negative effects on rhizobia growth. However, the greenhouse experiment found no evidence that A. petiolata leaf tissue additions affected nodule number, total nodule mass or average nodule size (all P> 0.85). Furthermore, A. petiolata did not appear to have a negative effect on the performance of A. bracteata, as the treatment with A. petiolata showed no difference in A. bracteata’s total biomass (P = 0.46) or growth (P> 0.85). Together, our results indicate that the compounds derived from A. petiolata might have allelopathic effects on rhizobia but only at high concentrations. Further experimentation will be carried out in 2013 to clarify the direct and indirect effects of garlic mustard on native legume performance.