Induced responses to herbivory in the Hawaiian endemic prickly poppy, Argemone glauca, and the Mexican poppy, Argemone mexicana

Abstract

Herbivory is an important factor in the evolutionary ecology of plants, and plants have many chemical and physical traits that reduce the negative effects herbivory has on plant fitness. Induced responses to herbivory is a now widely accepted ecological pattern, although most studies have examined responses in chemical defense traits, while physical defense traits and mechanisms of toleralnce have received less attention. We studied the effects of removal of 50% of leaf area and of jasmonic acid application on the induction of leaf latex, prickle density, leaf toughness, as well as mechanisms of tolerance in two sister species, Argemone glauca and Argemone mexicana. Potentially important factors of variation in defense and tolerance were also investigated, including ontogeny and genetic variation within and between populations. A. glauca significantly increased prickle densities in response to treatments, while A. mexicana increased leaf toughness and showed a higher ability to compensate for biomass loss than A. glauca. No treatments induced changes in leaf latex volume. Genetic variation between populations was detected in physical defense traits at constitutive levels, but not in inducible responses. This study is the first to induce prickle densities in a plant using jasmonic acid as a form of simulated herbivory, and is the first to induce responses in defense traits in a Hawaiian endemic. This study also highlights the divergence of defenses in two closely related species. Additional studies using natural herbivores are needed to confirm that these responses provide defense against herbivory.