Selection strengthens the relationship between plant diversity and the metabolic profile of <i>Plantago lanceolata</i>

Plant species diversity enhances community productivity, but how plant diversity impacts the metabolome of individual plants and the underlying eco-evolutionary processes remains unclear. This study investigated how plant species diversity and selection for growing in different diversity environments affect the leaf metabolome of Plantago lanceolata. We compared the metabolites of plants derived from those that had been selected in the 'Jena Experiment' for 17 yr in plant communities with differing plant diversity with the metabolites of naïve plants not subjected to this selection. The metabolic profiles of selected P. lanceolata plants were also compared after growing in experimental environments varying in plant diversity, soil history and community plant history. Volatile compound diversity in P. lanceolata decreased with plant species richness (SR), primarily due to phenotypic plasticity rather than selection. Soil history further strengthened this relationship. Conversely, non-volatile compound diversity increased with plant SR, but only in phytometers subjected to diversity-driven selection. These effects were more pronounced when plants shared soil-plant history with their community. In summary, our study revealed that both plastic and adaptive responses shape the metabolome of P. lanceolata in relation to plant diversity, with these effects becoming stronger as plant and soil communities mature.