EFFECTS OF PLANT-AGE, GENOTYPE, AND HERBIVORY ON PLANTAGO PERFORMANCE AND CHEMISTRY

Plant performance and chemistry may vary due to a variety of factors, such as plant genotype, environmental conditions, presence of herbivores, timing of herbivory, and species of herbivore. The relative importance of these factors, and how the plants respond,to them, may affect the dynamics of the plant population, as well as the insect herbivores feeding on those plants. To understand the relative importance of some of these factors on plant performance and chemistry, we used Plantago lanceolata L. (Plantaginaceae). In an experimental garden at Binghamton, New York, we examined the effects of plant age, plant genotype, and herbivory by generalist or specialist caterpillars on P. lanceolata. There were two parts to this experiment. In the first, we examined variation in nutritional quality (nitrogen) and defensive chemistry (the iridoid glycosides aucubin and catalpol) As a consequence of plant age, plant genotype, and leaf age class. We compared these parameters for a set of four pairs of plants of each of five genotypes that were not exposed to herbivores, and were harvested in early July (control-start plants) with another set harvested 6 wk later in mid-August (control-end plants). The older plants harvested in August (control-end plants) had concentrations of aucubin, catalpol, total iridoid glycosides, and nitrogen approximately one-half that of plants harvested 6 wk earlier. Leaf age affected all of these variables, and plant genotype influenced the iridoid glycoside variables but not the nitrogen concentration. Overall, leaf age explained twice as much of the variation in iridoid glycosides as did plant age, which accounted for twice as much variation as did plant genotype. The second part of the experiment compared the effects of herbivory, genotype, and leaf age on plant performance (leaf biomass, scape [flower stalk] biomass, and total biomass) and chemistry (nitrogen, protein, and iridoid glycoside concentration). We compared these measures for a set of four pairs of replicate plants of each of five genotypes, exposed to one of three herbivory treatments: no herbivory, herbivory by specialist caterpillars, and herbivory by_generalist caterpillars. The results of this experiment showed that herbivory had-little effect on plant performance, and there was no difference due to herbivory by the specialist Junonia coenia Hubner (Nymphalidae), compared to the generalist Spilosoma congrua Wlk. (Arctiidae). However, plant chemistry was significantly affected by herbivory. Herbivory by both caterpillar species induced iridoid glycosides and resulted in an increased concentration of catalpol and an increase in the proportion of catalpol relative to total iridoid glycosides in those plants exposed to herbivores compared to controls. In addition, plants exposed to specialist caterpillars had higher concentrations of catalpol and a higher proportion of-total iridoids that was catalpol than those exposed to generalist caterpillars. Overall, in this experiment, leaf age explained three times as much of the variation in iridoid glycosides as did plant genotype and herbivory. The results of the two parts of this experiment show that iridoid glycoside concentration in P. lanceolata leaves decreases over time, and that exposure to herbivores induces increased concentrations of iridoid glycosides.