Root chemistry in Populus tremuloides: effects of soil nutrients, defoliation, and genotype

Although genetic, environmental, and G x E effects on aboveground phytochemistry have been well documented in trembling aspen (Populus tremuloides), little work has focused on the same factors affecting tissues underground. Belowground plant defenses are likely important mediators of root-feeding herbivores that can strongly influence plant fitness. We used a common garden of potted aspen trees to explore the individual and interactive effects of soil nutrient availability, foliar damage, genotype, and their interactions, on concentrations of phytochemicals in aspen roots. Our common garden experiment employed 12 aspen genotypes that were planted into either low- or high-nutrient soil environments. Half of the trees were subjected to defoliation for two successive years, while the others were protected from damage. At the end of the growing season after the second defoliation, we harvested the trees to obtain root samples for which we assessed levels of phenolic glycosides, condensed tannins, nitrogen, and starch. Phenolic glycosides were most affected by genotype, while the other root phytochemicals were most responsive to soil nutrient conditions. The effects of defoliation were observed in interaction with soil nutrient environment and/or genotype. Interestingly, the effect of defoliation on phenolic glycosides was mediated by soil nutrients, whereas the effect of defoliation on condensed tannins was observed in concert with effects of both soil nutrients and genotype. Comparison of data from this study with an earlier, related study revealed that concentrations of phenolic glycosides and condensed tannins are lower in roots than leaves, and less responsive to defoliation. That soil nutrient environment affects root phytochemical concentrations is not unexpected given the intimate association of roots and soil, but the complex interactions between soil nutrients, aboveground damage, and genotype, and their effects on root phytochemistry, are intriguing. Variation in root chemistry could have wide-reaching effects on soil microbial communities, nutrient cycling, and herbivores. Additionally, the response of phytochemicals to damage across organs can link different, spatially separated herbivores as they use different parts of the same plant resource.