| The study of plant defenses against herbivores has generally focused on adult plants. In contrast, relatively little is known about seedling defense and how it might differ from that of adult plants. This is an important oversight because herbivory is likely to impact plant fitness most at the seedling stage. To address, I conducted a series of experiments to characterize seedling defense strategies in two widespread and common weeds, Plantago lanceolata and P. major (Plantaginaceae), species which produce terpenoid secondary compounds, iridoid glycosides, for defense. Three putative defense traits (constitutive secondary chemistry, induced secondary chemistry and compensatory growth) were measured in young plants across a range of ages (two to six weeks) to investigate early ontogenetic patterns in these traits. Constitutive iridoid glycoside concentrations were low in seedlings, undergoing a dramatic increase between four and six weeks of age. Damage by a specialist herbivore, the buckeye caterpillar (Junonia coenia, Nymphalidae) led to significant reductions in iridoid glycosides in four-week-old P. lanceolata plants, a pattern inconsistent with induced resistance to herbivores, but had no effect on two-week-old seedlings. Plantago lanceolata plants showed full shoot compensation for 50% seedling defoliation by buckeye caterpillars, although this occurred at the expense of root biomass in plants damaged at four weeks of age. Genetic variation was observed for constitutive secondary chemistry and ontogenetic patterns in constitutive secondary chemistry, but not for induction or compensation, suggesting that the two latter traits may be constrained in their capacity to evolve by natural selection. To put this work into a more meaningful community context, I conducted a series of experiments investigating how plant neighbors (potential competitors) influence ontogenetic patterns in plant defense. In the greenhouse, the presence of neighbors led to significant increases in iridoid glycoside concentrations, although these effects depended on plant age and neighbor identity. In the field, plant neighbors had no detectable effect on plant performance (survival and growth) or iridoid glycoside concentration, likely due to reductions in root competition compared to plants grown in pots in the greenhouse. My research provides novel insights into plant-herbivore dynamics and the evolution of plant defense by revealing dramatic ontogenetic patterns in plant defense. |
