HPLC analysis of cardenolide induction in Asclepias syriaca leaves and nectar following application of jasmonic acid
Plants produce an array of secondary defense compounds to combat herbivory. However, secondary compounds in nectar seem to serve a paradoxical function, since nectar is commonly used to attract insect pollinators. Some researchers have proposed that the presence of secondary compounds in nectar serves an adaptive purpose, acting as a deterrent towards inefficient pollinators or nectar-robbers. Others believe in a consequence-of-defense theory to explain this phenomenon, wherein the systemic production of secondary compounds throughout many plant structures may have inadvertently caused leakage into other structures not necessarily in danger of herbivory-in this case, nectar. The aim of this study was to understand the presence of specific secondary compounds called cardenolides in nectar in the common milkweed, Asclepias syriaca, and whether cardenolide identities between leaf and nectar samples were similar according to the consequence-of-defense theory. In an attempt to induce cardenolide production, we used exogenous jasmonic acid (JA), a compound previously shown to play a role in plant chemical defense pathways, and explored whether positive correlations existed between leaf and nectar cardenolide concentrations from the same plant via the consequence-of-defense theory. High performance liquid chromatography (HPLC) was used to analyze all samples. Results showed that for both nectar and leaf tissue, JA-treated and control-treated samples did not differ significantly in cardenolide levels. There was also no positive correlation between cardenolide levels in leaf and nectar samples of the same plant. Future studies should focus on a detailed .analysis of cardenolide identities to determine the presence of unique compounds in nectar, which . could shed light on the validity of the consequence-of-defense theory.