Condensed tannins (CTs) are widely distributed, polymeric phenolic compounds that mediate interactions between plants, herbivores and the environment. Within a species, CTs vary widely in concentration and composition, and are differentially affected by environmental factors such as herbivory. Herbivore damage is known to alter CT concentrations, yet little is known about how it affects CT structural properties (polymer chain length and relative subunit abundance) that are important determinants of CT bioactivity. We examined how insect folivory (by gypsy moth, Lymantria dispar) and simulated mammal herbivory (by removal of stem and leaf tissue with pliers) affect CT concentration and composition in the foliage of multiple trembling aspen (Populus tremuloides) genotypes. Foliage generated following damage was analyzed for CTs using thiolysis in conjunction with ultra high performance liquid chromatography-mass spectrometry (UHPLC/MS). CT concentrations, polymer chain lengths, and relative subunit abundances were compared across treatments using ANOVA.
Results/Conclusions
Insect and mammal damage decreased foliar CT concentrations by 34% and 56%, respectively. Insect damage affected CT composition by altering the relative abundance of catechin extender units, with the directions of the effect differing among genotypes. In contrast, damage to leaves and stems exerted a stronger effect, altering polymer chain length and relative abundance of gallocatechin extender units (with effects differing among genotypes), and increasing the relative abundance of catechin terminal units by 12% across all genotypes. Gallocatechin subunits are more hydroxylated than catechin subunits. Since hydroxylation and chain length can affect the biological properties of CTs in vitro, the effects of herbivore damage on CT composition seem likely to modulate the ecological effects of CTs. Furthermore, the preponderance of damage × genotype interactions observed herein suggest that herbivores may drive the evolution of plant-herbivore associations through effects on CT composition as well as concentration.