Multiple factors determine the fitness benefits for plants of rhizobia bacteria associations. Even for congeneric species growing in the same habitat, small difference in flavonoid chemistry may alter the degree to which closely related plant species form and maintain rhizobia mutualisms. These differences in one class of biochemical metabolite may then alter investment in additional secondary metabolites in ways that influence herbivory, growth, fitness, and mutualistic relationships. Nonetheless, most studies address variation in rhizobia benefits within individual species and often ignore the third trophic level, limiting our ability to understand how such interactions evolve. To estimate within and across species variation in rhizobial nodulation and its contribution plant growth and anti-herbivore defense, we performed greenhouse experiments with three prairie plants: Baptisia alba, B. australis, and B. tinctoria (Thermopsidae:Fabaceae). We germinated seeds of all 3 species with and without rhizobia to determine if rhizobia improved germination. Then, we grew seedlings on either sterile soil (control, RH0) or in rhiozobia-amended soil (treatment, RH1) and monitored growth over a 5- week period. We used leaves from RH0 and RH1 plants to conducted herbivory assays with specialist caterpillars (Erynnis baptisiae: Hesperiidae) to assess if nodulation altered herbivore performance.
Results/Conclusions
We found that rhizobia did not alter time to seed germination, but that root nodulation delayed early seedling establishment in B. australis and B. tinctoria (Germination of B. alba was too low to use this species in analyses). RH1 plants took 2 days longer to reach their first leaflet stage (F1,106 = 10.44, P = 0.0017). However, plants that did nodulate offset this delay by increasing the rate of above ground growth, such that we did not find that RH0 and RH1 plants differed in overall seedling establishment measured by root and above ground growth rates for 5 weeks (approximate time to 3rd leaflet stage). We observed plant species difference in herbivore performance. Specifically, leaf consumption and assimilation were higher for caterpillars feeding on B. tinctoria (F1,37 = 17.43, P = 0.0002 and F1,37 =8.38, P = 0.0066, respectively). Parasitism of field collected Erynnis caterpillars reduced our sample size for detecting treatment effects: While assays did not reveal differences in herbivory with RH treatment, we found a marginal negative correlation between root nodulation and assimilation by Erynnis caterpillars (R2 = 0.2145, P = 0.0611), suggesting that RH1 plants may increase both growth and anti-herbivore defense.