Plant responses to arbuscular mycorrhizal (AM) fungi can vary widely across functional groups, with functionally similar species often displaying similar responses to AM symbioses. While plant functional group may explain much of the variation in mycorrhizal associations of plants within a community, variation in responses within genera has been observed and may be explained in part by phylogenetic relatedness. Milkweeds (Asclepias) are a diverse genus of plants in the family Apocynaceae, with nearly 130 species found in North America. Although milkweed research has increased dramatically over the past two decades, relatively little research exists examining the relationship between AM fungi and milkweeds. Understanding the responsiveness, or dependence, of milkweed species on this symbiosis may be vital to successful restoration efforts that include these species.
Our research was conducted at greenhouse facilities at Oklahoma State University, Stillwater, OK. Thirty-one species of milkweed found across the United States were selected to assess the relative mycorrhizal responsiveness (rMR). All seedlings were grown in sterile soil collected from native prairie sites, and half of the plants were inoculated with 20 g-1 of whole prairie soil with intact microbial communities. End of season biomass of mycorrhizal and non-mycorrhizal plants was used to calculate rMR.
Results/Conclusions
rMR of selected milkweeds varied greatly between species, ranging from 36 to 97%. This was unexpected, based on previous research assessing the variation in responsiveness within genera. However, thirty of thirty-one species of milkweed produced significantly greater (p ≤ 0.05) biomass when inoculated with whole prairie soil. This suggests that while there is remarkable variation in the degree of responsiveness within the genus, most species receive benefits from the association with AM fungi. Our study is the first to assess rMR of a large number of milkweed species, most of which are known to be consumed by monarch larvae. We found that mycorrhizal colonization of the roots was not correlated to rMR. Evaluation of the phylogenetic component of variation in rMR data suggests that relatedness is an important determinant of responsiveness to AM fungi. Our research indicates the genus Asclepias is broadly responsive to AM fungi, however, interspecific variation is apparent.