Changes in plant quality and defenses as a function of plant ontogeny

Background/Question/Methods Despite the fact that multiple traits known to influence species interactions change over the course of an organisms’ ontogeny (i.e. throughout their development), ontogeny has been rarely studied as an intrinsic factor able to influence such interactions. Plant ontogeny has been shown to affect the expression of numerous plant traits relevant to herbivores such as nutritional content as well as physical and chemical defenses. However, we know little about how these plant traits changes across multiple developmental stages of a plant species. To date, few empirical studies have examined the influence of plant ontogeny in explaining variation in plant defenses. However, most of these studies usually compare two ontogenetic stages leading to the support of hypotheses predicting a linear decrease or increase as plants growth. However, a recent review by Boege and Marquis (2005) predicted a non-linear relationship between plant age and defenses which emphasizes the need to evaluate plant defense strategies at multiple developmental stages. In order to evaluate how plant traits relevant to herbivores can change over plant ontogeny, I measured plant biomass, root:shoot ratio, nutritional quality, and constitutive concentrations of chemical defenses (iridoid glycosides) at seven distinct ontogenetic stages of Plantago lanceolata (Plantaginaceae): 4, 6, 9, 11, 14, 16 and 19 week-old. The first two stages correspond to seedling stages, the second two with juvenile stages, the third two with reproductive stages, and the last one with a post-reproductive stage. All plant stages were analyzed simultaneously in order to control for potential environmental factors that could influence plant performance such as photoperiod and temperature.

Results/Conclusions Plant age significantly influenced total aboveground biomass (p<.0001), root:shoot ratios (p<.0001), water content (p<.0001), carbon:nitrogen ratios (p<.001), and plant chemical defenses (p<.0001). Interestingly, all of these traits showed a non-linear trend with plant age. For example, plant biomass and quality increased over time reaching its highest value for 14 and 11wks-old plants respectively, and decreasing afterwards. Constitutive defenses also varied non-linearly, showing a decrease from 4wks to older seedling stages and a dramatic increase from 11wks to 19wks-old plants. Changes in plant defenses and nutritional value are known to influence insect herbivores by directly altering herbivore performance and/or indirectly altering herbivore predation risk. Thus, the variation in plant quality and defenses reported here in relation to plant ontogeny is expected to play a significant role in mediating plant-herbivore and herbivore-predator interactions.