Temporal and species-specific variation in metabolomic responses to drought in European elms

Introduction European elms ( Ulmus minor , U. laevis and U. glabra ) are strongly constrained by biotic stress, and increasing summer drought may further limit their performance under climate change. Here, we investigated drought-driven changes in the primary and secondary metabolism of leaves in the three native European elm species grown under semi-field conditions and two watering regimes. Methods Leaves of the three elm species were analyzed using untargeted LC–MS and GC–MS metabolomic profiling. The analyses were complemented by quantification of non-structural carbohydrates and major phenolic classes, including total phenolics, flavonoids, and condensed tannins. The study evaluated the effects of sampling date, tree species, and watering regime on metabolite composition to determine the relative influence of drought and temporal dynamics. Results and discussion The metabolomic analyses revealed pronounced species-specific differences in both non-volatile and volatile metabolite profiles. GC–MS data were dominated by typical green-leaf volatiles (e.g., (E)-2-hexenal and (Z)-3-hexenyl acetate) and terpenes, which were generally more pronounced in U. glabra and U. laevis than in U. minor . Overall, species identity and sampling date explained a substantial proportion of the metabolic variation, while drought effects were moderate but species-specific: multivariate metabolomic separation was strongest in U. glabra , whereas U. laevis showed the clearest drought-associated increase in total phenolics, flavonoids and condensed tannins, and U. minor remained the most stable. These results indicate contrasting drought-response strategies among European elms; however, stronger metabolic responses do not necessarily reflect higher drought tolerance. Rather, U. minor appears to maintain greater metabolic stability under drought, whereas the pronounced changes observed in U. laevis likely reflect higher sensitivity to water deficit. Furthermore, the strong effect of sampling date highlights the importance of temporal dynamics in shaping plant responses to drought.