Induced Responses to Herbivory and Jasmonate in Three Milkweed Species
We studied constitutive and induced defensive traits (latex exudation, cardenolides, proteases, and C/N ratio) and resistance to monarch caterpillars (Danaus plexippus) in three closely related milkweed species (Asclepias angustifolia, A. barjoniifolia and A. fascicularis). All traits showed significant induction in at least one of the species. Jasmonate application only partially mimicked the effect of monarch feeding. We found some correspondence between latex and cardenolide content and reduced larval growth. Larvae fed cut leaves of A. angustifolia grew better than larvae fed intact plants. Addition of the cardenolide digitoxin to cut leaves reduced larval growth but ouabain (at the same concentration) had no effect. We, thus, confirm that latex and cardenolides are major defenses in milkweeds, effective against a specialist herbivore. Other traits such as proteases and C/N ratio additionally may be integrated in the defense scheme of those plants. Induction seems to play an important role in plants that have an intermediate level of defense, and we advocate incorporating induction as an additional axis of the plant defense syndrome hypothesis.
KeywordsMultiple defenses Secondary metabolites Latex Cardenolides Monarch (Danaus plexippus) Proteases Asclepias
We thank Amy Hastings for laboratory assistance, Rayko Halitschke for help with chemical analysis, Jennifer Thaler for providing jasmonic acid, Susan Cook, Rayko Halitschke and Mike Stastny for comments on the manuscript, and Steve Malcolm for providing seeds of A. barjoniifolia. Chemical analyses were conducted in the Cornell Chemical Ecology Core Facility, with support from Paul Feeny, New Life Sciences Initiative, College of Agriculture and Life Sciences, Center for a Sustainable Future, Boyce Thompson Institute, and Departments of Ecology & Evolutionary Biology, Neurobiology & Behavior, Entomology, Plant Biology, and Horticulture. Our research was supported by NSF-DEB 0447550 to AAA, and postdoctoral fellowship from Swiss National Science Foundation PA0033-121483 to SR.
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