Geography and associated bioclimatic factors differentially affect leaf phenolics in three ivy species (Hedera L.) across the Iberian Peninsula

The biosynthesis of secondary metabolites in plants, especially that of phenolic compounds, is stimulated to protect against several environmental stress factors such as cold temperatures, drought, and UV-irradiance. As a result, when a species occurs under different climatic conditions, differences in phenolic accumulation are expected across species distribution in response to the environmental cues. However, our understanding of phenolic compounds' natural variation is limited, as most of our knowledge on secondary metabolite biosynthesis stems from experimental studies conducted under controlled conditions. In this study we analyze phenolic content and its relation to climatic and geographic variation in three closely related Hedera species (H. helix, H. hibernica and H. iberica) across their southwestern range limits in the Iberian Peninsula (82 populations, 401 individuals). The Iberian Peninsula concentrates the highest global species richness of Hedera, with the three species sharing range boundaries along the latitudinal and longitudinal climatic gradient of the region. We found that the three species exhibited different climatic and geographic patterns of phenolic content variation in the study area. The phenolic production in H. helix increased with elevation in relation to the decrease of temperature and the increase of temperature contrast, whereas in H. hibernica varies with latitude in relation to summer temperature and precipitation regimes, increasing in areas with no summer drought. In contrast, we did not find any environmental variables associated with phenolic content in H. iberica, likely due to its narrow geographic and climatic range and a higher influence of microclimatic conditions. Although the three Hedera species are closely related, our results suggest that leaf phenolic production may be triggered by different environmental conditions in each species. Our study underscores the species-specific nature of phenolic compounds' role in plant stress response.