Plant age and herbivore induced defenses: what matters the most?

QUINTERO C

Boulder, Colorado

Encuentro; Forth Annual Guild of Rocky Mountain Ecologists and Evolutionary Biologists; 2008

University of Colorado at Boulder

Plant induced defenses following herbivore damage can change the nutritional quality and palatability of a host plant for future generations of herbivores. On the other hand, throughout plant ontogeny physiological constraints during plant development can also lead to variable plant quality and palatability to herbivore insects. However, to date, the relative importance of herbivore induced responses and plant developmental constraints in shaping plant defenses remains unknown. I studied the relative importance of plant age versus history of damage using Plantago lanceolata during Spring-Summer 2006. Plants were subjected to herbivory at two different ages (9- and 14-weeks-old). Plants at the age of 9 weeks (240 plants) were randomly assigned to one of three herbivory treatments: “C” = control, “S” = specialist or “G” = generalist herbivore. Five weeks later, plants at the age of 14 weeks (180 plants) were subjected to a second herbivory event by randomly assigning previous plant treatments to one of two new herbivory treatments: “C” and “S”. Before the 1st and 2nd generation of damage occurred, I harvested 20 plants per treatment to estimate plant initial chemistry. To test for induction of Iridoid glycosides (IGs), 20 plants per treatment were harvested one week after 1st and 2nd herbivore damage events. Concentration of IGs on leaf tissue was quantified using gas chromatography. History of damage did not affect plant concentration of IGs. Undamaged plants, as well as plants damaged before by specialist or generalist caterpillars, showed similar concentrations of aucubin and catalpol one week after the second herbivore event. Plant age, on the other hand, significantly affected induce resistance: while plants damaged by specialist caterpillars at 9-weeks old showed a significant decrease in plant defenses, no difference was observed one week after damage for 14 weeks-old plants damaged by the same herbivore species. These results suggest that although plant induced responses after herbivore damage do influence plant defensive chemistry leading to a bigger individual to individual variation within a plant-age group, plant ontogeny can play a more drastic role mediating the magnitude of plant ability to respond to damage.