Studies of the importance of root mutualists to aboveground organisms often examine one type of soil microbe with one aboveground organism. Yet, bacteria and fungi frequently colonize plant roots simultaneously, and plants provide for multiple organisms. Our research examined these microbes in concert for the annual legume Partridge Pea (Chamaecrista fasciculata: Fabaceae) to estimate their role in myrmecophilous plant-insect interaction. We grew seedlings in a greenhouse factorial experiment with and without rhizobia (RH) and mycorrhizae (M) soil inoculum (sterile soil control, RH, M, RH+M treatments). In field surveys, we counted ants, herbivores, extra-floral nectary (EFN) visitors, and pollinators, and correlated these with plant, leaf, EFN number, sugar concentration, and root colonization by both microbes. In greenhouse experiments, we measured plant height, leaf number and length, above and belowground biomass production, leaf chlorophyll, and EFN sugar concentrations. We used greenhouse leaf tissue in bioassay trials with field collected Melanoplus bivittatus (Orthopera:Acrididae).
Results/Conclusions
Rhizobia increased EFNs/leaf in both investigations. Significant negative correlations between plant size and root nodulation (field plants) and lower leaf production in RH and RH+M (greenhouse plants) suggest nitrogen investment from rhizobia into defense. Mycorrhizal colonization didn’t appear to impact plant growth. Greenhouse plants with higher leaf water content produced less concentrated EFN exudate. Herbivore densities didn’t correlate with ant density. However, nectar stealers declined with high ant density (R2 = 0.15, P = 0.0498). Bumblebees didn’t visit flowers with ants. Ants appeared to decline with EFN sugar concentration (field plants); likely responding to lower nectary exudate volume (greater water content likely produced larger volumes but dilute exudates). Mycorrhizae may have increased volumes of EFN nectar, accounting for positive correlations between ants and mycorrhizae vesicles. Rhizobia nodulation increased Melanoplus herbivory in bioassays. Both mutualists may be necessary for this ant-plant: Rhizobia promote nectary construction and mycorrhizae promote nectary exudate production.