Evolution of root exudate composition in ecologically-contrasting Helianthus
Plant roots exude numerous metabolites into the soil which influence nutrient availability. Although root exudate composition is expected to be under selection in low fertility soils both quantitatively and qualitatively, few studies have tested this hypothesis in a phylogenetic framework. In this study, we examined the root exudates of six Helianthus species chosen as phylogenetically-independent contrasts with respect to native soil nutrient availability. Plants were grown under controlled conditions to the three-leaf-pair stage, and then treated with either high or low nutrient treatments for five days. Gas chromatography-mass spectrometry was used for analysis of root exudates.
Overall, we detected 37 metabolites across species. We found that species native to low nutrient soils have constitutively high levels of root exudation for nearly all metabolites detected, regardless of nutrient availability. As expected, all species exhibited higher exudation of carboxylic acids under the low nutrient treatment than under the high nutrient treatment. However, species native to high nutrient soils also responded to low nutrient treatments with higher exudation of numerous other metabolites, potentially as a mechanism to directly or indirectly increase soil nutrient availability. Overall, the consistent evolutionary divergences detected among species native to low nutrient versus high nutrient soils provide evidence for the adaptive value of root exudate abundance, and plasticity in root exudation, in contrasting soil environments.