Effects of the Timing of Herbivory on Plant Defense Induction and Insect Performance in Ribwort Plantain (Plantago lanceolata L.

Abstract Plants often are exposed to antagonistic and sym-biotic organisms both aboveground and belowground. Interactions between above- and belowground organisms may occur either simultaneously or sequentially, and jointly can determine plant responses to future enemies. However, little is known about time-dependency of such aboveground-belowground interactions. We examined how the timing of a 24 h period of aboveground herbivory by Spodoptera exigua (1–8 d prior to later arriving conspecifics) influenced the re-sponse of Plantago lanceolata and the performance of later arriving conspecifics. We also examined whether these in-duced responses were modulated by the arbuscular mycorrhi-zal fungus (AMF) Funneliformis mosseae. The amount of leaf area consumed by later arriving herbivores decreased with time after induction by early herbivores.Mycorrhizal infection reduced the relative growth rate (RGR) of later arriving her-bivores, associated with a reduction in efficiency of conver-sion of ingested food rather than a reduction in relative con-sumption rates. In non-mycorrhizal plants, leaf concentrations of the defense compound catalpol showed a linear two-fold increase during the eight days following early herbivory. By contrast, mycorrhizal plants already had elevated levels of leaf catalpol prior to their exposure to early herbivory and did not show any further increase following herbivory. These results indicate that AMF resulted in a systemic induction, rather than priming of these defenses. AMF infection significantly re-duced shoot biomass of Plantago lanceolata. We conclude that plant responses to future herbivores are not only influ-enced by exposure to prior aboveground and belowground organisms, but also by when these prior organisms arrive and interact