Lignoid chemical defenses in the freshwater macrophyte Saururus cernuus

Chemical defense against herbivores has rarely been investigated for freshw ater plants, possibly due to the common misconception that herbivory on aqu atic macrophytes is low and would not select for chemical defenses. In prev ious work, the freshwater angiosperm Saururus cernuus was shown to be a low preference food for omnivorous crayfish despite its high nutrient value an d relatively soft texture. We used feeding by the crayfish Procambarus clar kii to guide fractionation of the deterrent lipid-soluble extract of this p lant, leading to the identification of seven deterrent lignoid metabolites, (-)-licarin A, (+)-saucernetin, (-)-dihydroguaiaretic acid, (-)-sauriols A and B, (-)-saucerneol, and (-)-saucerneol methyl ether. Lignans have been implicated in terrestrial plant chemical defenses as insect growth inhibito rs, insect toxins, nematocides, antibacterial, and antifungal agents. Howev er, these activities have rarely been demonstrated using ecologically relev ant methodologies in terrestrial systems, and never before in freshwater sy stems. The widespread nature of lignans amongst very distantly related plan ts, along with their rich diversity of molecular structure, suggests that t hey could play a large role in mediating plant-herbivore interactions. In a ddition to the lignoid compounds we identified, there were other compounds present in low concentration or unstable compounds that were deterrent, tha t did not appear to be lignans, but that we were unable to identify. This p lant thus appears to be defended by a complex mixture of natural products.