Characterization of the chemical defenses of Sagittaria graminea, a freshwater plant, against crayfish herbivory
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Herbivores powerfully impact community structure by altering biomass, species richness, and succession, which cause many plants to evolve chemical defenses against them. Chemical defenses have been well studied in marine and terrestrial systems, but studies focusing on freshwater chemical ecology have only recently begun to increase in number. The freshwater macrophyte Sagittaria graminea is relatively nutritious and found in many of the same habitat types as crayfish, which can consume large amounts of biomass. We looked to chemical defenses to explain how S. graminea can maintain its populations despite the presence of crayfish. In this study, we aimed to characterize the deterrent secondary metabolite(s) found in S. graminea through bioassay-guided fractionation. Additionally, we compared deterrence of stem and leaf regions to test for differential allocation of resources within individuals. Significant deterrence by several distinct fractions of S. graminea extracts indicates that more than one chemical is contributing to S. graminea’s defenses. Additionally, one of these fractions contains chemical characteristics similar to other identified freshwater plant defensive chemicals. We determined that the deterrent chemicals have relatively low polarity. However, the lack of significant differences between consumption of the stem and leaf region of S. graminea indicates that defenses are equally allocated throughout the individuals, despite the easier accessibility of the stem to crayfish in the wild. Characterization of the deterrent chemical(s) and investigation of differential resource allocation contribute to a relatively unexplored area of chemical ecology by providing insight into the structure and regulation of a widely distributed macrophyte’s defense.