Background/Question/Methods Plants can minimize the negative impact of herbivory via two well recognized mechanisms: tolerance and resistance. Such herbivore induced responses can be influenced by numerous extrinsic and intrinsic factors such as resource availability, herbivore identity, plant genotype, and plant developmental constraints. Among these, the role of plant ontogeny in influencing the ability of plants to compensate or induce defenses has been the least explored. Thus, our objective was to assess how plant ontogeny can alter short and long term responses to herbivory (i.e. tolerance and resistance) using multiple developmental stages of Ribwort plantain (Plantago lanceolata, Plantaginaceae). We used five ontogenetic stages of P. lanceolata: 3, 5, 10, 12, and 18 week-old (n=90 plants/age), ranging from seedlings to reproductive plants. Treatment plants (n=45 plants/age) were simultaneously damaged by one to eight Buckeye caterpillars (Junonia coenia, Nymphalidae) until they consumed ~30% of the available leaf tissue. To assess short and long term responses in compensatory regrowth (i.e. tolerance) and induction of iridoid glycosides (i.e. resistance), control and treatment plants were harvested one, three and five weeks after damage (n=15 plants/age/treat/harvest time).
Results/Conclusions Plant age significantly influenced the ability of P. lanceolata to tolerate or induce defenses after damage. In terms of compensatory regrowth, while seedling stages did completely compensate for the lost biomass early after damage, some older stages were unable to compensate for herbivory even after 5 weeks following herbivore removal. Similarly, induced defenses also varied with ontogeny; with seedlings showing no difference in iridoid glycosides between damaged and control plants, while older stages switched from induced susceptibility to induced resistance. In both cases, three way-ANOVAs revealed a significant effect of plant age (p < 0.0001), herbivory treatment (p < 0.0001) and harvesting time (p < 0.0001), as well as a significant interactions between plant age and harvesting time (p < 0.01) and plant age and herbivore treatment (p < 0.05). These results demonstrate that plant responses to herbivory change considerably with plant age and time elapsed following damage. Herbivore induced plant responses after damage may influence future herbivores by directly altering herbivore host selection and performance, and/or indirectly altering herbivore predation risk. Thus, the variation in plant tolerance and resistance reported here as a function of plant ontogeny is expected to play a significant role in mediating multitrophic interactions.