Chemical mediation of Penstemon-herbivore interactions: A comparison among Colorado populations

Background/Question/Methods

Plants produce a variety of chemical compounds that can function as a defense against natural enemies, such as herbivores.  Plant chemistry depends on several abiotic and biotic factors including nutrient availability, plant genotype, and previous damage.  Therefore, plants in the same species may vary widely in their chemical defenses and such variation can be evident at the population level, as well as the individual level.  Focusing on the native Colorado wildflower, Penstemon virgatus, my research assesses within and among population variation in defensive compounds for three tissue types: leaves, stems and flowers.  Penstemon species are known to contain iridoid glycosides (IGs), defensive compounds easily detected using gas chromatography.  I collected plants for chemical analysis from six populations across Colorado, including two distinct populations within Rocky Mountain National Park.  In the field, I quantified the amount of damage due to invertebrate herbivores visible on each focal plant, allowing me to quantify the relationship between plant chemistry and herbivore damage.   

Results/Conclusions

Chemical analyses suggest that individual plants are able to allocate different IGs into different tissue types depending on the location.  Often, reproductive tissues (flowers) contain higher amounts of the IG catalpol than seen in leaves, yet leaves contained extremely high amounts of the IG scutellarioside (upwards of 40% dry weight).  Furthermore, there is a wide variation in total amounts of chemical defense among populations, which may be due to the varying abiotic factors at each location, as all populations showed very low amounts of herbivory.  By collecting data on natural Penstemon virgatus IG variation, I can predict how these native plants interact with local herbivores, particularly insects, and how these sources of variation may scale up to higher trophic levels.