The scent of survival in a warming world: how monoterpenes drive thermal adaptation in thyme

Andreas Havbro Faber , John D Thompson, Perrine Gauthier, Bodil Kirstine Ehlers

1 Monoterpenes are key plant secondary metabolites with well known defensive and ecological functions, yet their role in abiotic stress tolerance remains poorly understood. In many Mediterranean plants, monoterpene composition varies markedly within and among species and is associated with climatic gradients, suggesting that these compounds may mediate plant responses to extreme heat.
2 We investigated two locally adapted ecotypes of Thymus vulgaris that differ in monoterpene chemistry. Phenolic ecotypes dominate shallow, rocky habitats with hot, dry summers and mild winters, whereas non phenolic ecotypes occur in deeper soils exposed to more severe winter freezing. Although previous transplant experiments show strong local adaptation and stable geographic distributions despite high gene flow, the ecophysiological mechanisms linking monoterpene variation to climatic tolerance remain unknown.
3 We quantified heat tolerance in both ecotypes using controlled experiments that measured photosynthetic thermal limits, and combined these with field based assessments of physiological performance, mortality, and long term changes in ecotype composition. Using a thermal death time (TDT) framework, we predicted when each ecotype would experience photosynthetic failure under natural conditions and related these predictions to observed mortality during recent periods of intensified summer heat and drought.
4 Heat tolerance diverged between ecotypes only after heat acclimation, with the phenolic ecotype substantially increasing its thermal limits relative to the non phenolic ecotype. This indicates that monoterpene chemistry primarily affects fitness through enhanced tolerance to extreme temperatures rather than through performance differences under benign conditions. However, despite its superior heat tolerance, the phenolic ecotype experienced elevated mortality in its warm native habitats, suggesting that recent summer temperatures may already exceed the physiological thresholds of the phenolic ecotype.
5 Synthesis. Understanding how plant functional traits mediate responses to climatic extremes is essential for predicting vegetation dynamics under climate change. Our results show that intraspecific variation in secondary chemistry can enhance physiological heat tolerance via increased acclimation capacity, yet rapid climatic warming may still overwhelm these trait based advantages. By integrating plant chemistry, physiological thresholds, and long term demographic changes, this study advances our understanding of how functional traits shape plant vulnerability and resilience in increasingly extreme environments