The fungal fast lane: common mycorrhizal networks extend bioactive zones of allelochemicals in soils

Abstract

Allelopathy, a phenomenon where compounds produced by one plant limit the growth of surrounding plants, is a controversially discussed factor in plant-plant interactions with great significance for plant community structure. Common mycorrhizal networks (CMNs) form belowground networks that interconnect multiple plant species; yet these networks are typically ignored in studies of allelopathy. We tested the hypothesis that CMNs facilitate transport of allelochemicals from supplier to target plants, thereby affecting allelopathic interactions. We analyzed accumulation of a model allelopathic substance, the herbicide imazamox, and two allelopathic thiophenes released from Tagetes tenuifolia roots, by diffusion through soil and CMNs. We also conducted bioassays to determine how the accumulated substances affected plant growth. All compounds accumulated to greater levels in target soils with CMNs as opposed to soils without CMNs. This increased accumulation was associated with reduced growth of target plants in soils with CMNs. Our results show that CMNs support transfer of allelochemicals from supplier to target plants and thus lead to allelochemical accumulation at levels that could not be reached by diffusion through soil alone. We conclude that CMNs expand the bioactive zones of allelochemicals in natural environments, with significant implications for interspecies chemical interactions in plant communities.

Figure 1

(a) Exploded view of H-pot construction, showing location of large nurse plants, small bioassay plant, the hole where imazamox was introduced to the soil (indicated by droplet symbols), and the four layers of mesh separating pot halves. (b) Experimental design, with solid steel plates indicating that soil was packed into the perforations in the steel plate, while open steel plates were left open creating an air gap. Arrows indicate that plates were moved daily.

Figure 2

(a) Design of Experiment 2 showing root exclusion compartments (REC) as dashed lines, location of Tagetes plant, and in situ extraction tubing in gray. The CMN is indicated by black lines, and rotation of the REC prevents CMN formation inside. (b–d) Results from Experiment 2 (means ± SE). Bars with different letters indicate significant differences at α = 0.05. (b) Levels of α-T in in situ extraction tubing inside RECs (N = 10). (c) Levels of BBT in in situ extraction tubing inside RECs (N = 10). (d) Above ground biomass of bioassay plants in REC soil after initial harvest (N = 11).

Figure 3. Results from Experiment 1 ± (means SE).

Gray bars indicate bulk soil flow, and open bars indicate no bulk soil flow. Bars with different letters indicate significant differences at α = 0.05 with a one-factor (CMN) model. (a) Above ground biomass at harvest 2 (N = 5). (b) Above ground biomass at harvest 3 (N = 5). (c) Imazamox concentrations in leaves at harvest 3 (N = 4–5).