An organism’s phenotype determines in large part how it interacts with the world around it. While many forces shape organism phenotype, among the most important are organism genotype, abiotic conditions, and interspecific interactions. In the work presented here, we describe the relative effects of each of these factors on the defensive phenotype of Asclepias syriaca (common milkweed). Specifically, we explore the main and interactive effects of family identity, atmospheric CO₂ concentration, and the presence or absence of mycorrhizal fungi on the defensive phenotype of A. syriaca. Asclepias syriaca is common to the Eastern United States, hosts arbuscular mycorrhizal fungi, and is prey to many specialist insect herbivores. A. syriaca produces many putative defenses, including cardenolides, latex, trichomes, and tough leaves. We germinated 80 individuals each of five full-sibling families of Asclepias syriaca originating from Northern Michigan. In a double split plot design, replicates of each family were grown in the presence of two treatments: ambient and elevated CO₂ concentrations (380 and 760 ppm, respectively), in soil with mycorrhizal inoculum or a control soil wash. Plants were destructively harvested, their multi-trait defensive phenotype assessed, and roots were examined for mycorrhizal colonization. Data were analyzed using SAS ProcMixed models.

Results/Conclusions

We found that family identity, CO₂ concentration, and mycorrhizal interactions influenced plant defensive phenotype, although to different extents. Elevated CO₂ increased plant defense by increasing leaf thickness and trichome density, while mycorrhizal fungi increased leaf trichome density. Overall, family effects outweighed those of either treatment; leaf thickness, trichome density, latex production, and the number of cardenolide compounds produced varied widely among families. Furthermore, family identity interacted with CO₂ concentration to affect the production of latex and with mycorrhizal infection to affect trichome density. We conclude that although biotic interactions and changing abiotic conditions affect A. syriaca phenotype, intraspecific genetic variation generates the largest source of variation in defensive phenotype. Significant interactions between plant genotype and other ecological forces suggest that natural selection may result in genotypic change in A. syriaca populations under some scenarios of environmental change.