Effects of genotype and environment on the composition of foliar condensed tannins in aspen (Populus tremuloides)

Background/Question/Methods . Condensed tannins (CTs) in plant foliage can influence herbivore performance through their role as defense agents, and soil processes through their effects on microbial activity and nutrient cycling. Many studies show that CT concentrations vary within and among species, and in response to environmental factors. However, effects of these variables on CT polymer composition (e.g., chain length and subunit type) are usually neglected, even though polymer composition is expected to determine the chemical/biological properties of CTs. We investigated how genotype and environment affect total CT concentration, polymer chain length and subunit abundance in trembling aspen (Populus tremuloides) foliage. We measured CT variation (1) among 13 genotypes grown in a common garden, and (2) in response to elevated CO₂ at the aspen Free Air Carbon Dioxide Enrichment site (Rhinelander, WI, USA). Total CTs were quantified colorimetrically by a standard acid butanol assay. CT composition was analyzed by extracting dried plant tissue into methanol, and decomposing CTs into their constituent monomers by reaction with benzyl mercaptan. Monomers were separated, identified, and quantified by UHPLC/MS. The polymer chain lengths and molar concentrations of subunits were compared among genotypes and environmental treatments using ANOVA.

Results/Conclusions . Aspen foliar CTs were primarily composed of four subunits: catechin, epicatechin, gallocatechin, and epigallocatechin, each differing by a single hydroxyl group and/or orientation at one steric center. Among genotypes, polymer chain lengths varied from 5 to 11 subunits, and concentrations of extender subunits varied from 6 (epigallocatechin) to 150 (epicatechin) nmol/mg leaf. A weak correlation between total CTs and the sums of the dominant monomers (R² = 0.22, N = 55, P < 0.01) suggests that an assay of total CTs does not capture all qualitative variations in CTs. Elevated CO₂ increased overall CT concentrations, but did not meaningfully affect polymer chain lengths or molar subunit concentrations. Our study shows that polymer composition varies among genotypes, but not in response to (some) environmental factors, in aspen. Because the composition of CTs is expected to affect their chemical properties, differences in CT composition are likely to influence the role of these compounds as herbivore defenses and mediators of plant-soil interactions. Future studies should evaluate the ecological consequences of qualitative variations in CT composition, and elucidate patterns in how composition is affected by various environmental factors.