Short-term high-temperature stress enhances trait plasticity and phytochemical yield in the Southern African medicinal shrub Lippia javanica

Precipitation, vapour pressure deficit and temperature are the primary drivers of Lippia javanica 's distribution in South Africa Heat stress promotes protective responses in L. javanica that shift plant resource allocation from growth to defence Heat stress promotes cuticle thickening and mesophyll compaction. Heat stress enhances essential oil, phenolic, and flavonoid accumulation. Trichome density and length increase under high temperature exposure. Lippia javanica (Burm. f.) Spreng., a culturally significant medicinal shrub in African traditional healthcare, is increasingly valued for its therapeutic potential under changing climatic conditions. However, its resilience to elevated temperatures remains poorly understood. This study integrated species distribution modelling (SDM) with controlled-environment experimentation to assess both the climatic suitability and thermal acclimation capacity of L. javanica . SDMs based on 1502 occurrence records identified precipitation, vapour pressure deficit, and temperature seasonality as the primary drivers of its distribution across South Africa’s summer rainfall zones. Informed by these predictors, mature plants were exposed to episodic high-temperature regimes (47/37 °C day/night) for 48, 96, and 144 h. Heat stress reduced leaf number but induced compensatory increases in leaf area, petiole diameter, and trichome development. Anatomical adaptations included cuticle thickening, palisade compaction, and deposition of dark-staining compounds in the mesophyll, indicative of enhanced protective and metabolic responses. Phytochemical analyses revealed substantial increases in essential oil yield (66.7%) and in total phenolic and flavonoid content (67.3% and 69.8%, respectively). These coordinated morphological, anatomical, and biochemical responses indicate capacity for short term acclimation in L. javanica , with heat stress eliciting pronounced shifts in secondary metabolite accumulation consistent with protective and stress-response functions rather than inferred medicinal optimisation. Whether these changes translate into altered or enhanced medicinal value remains unknown and requires targeted phytochemical and bioactivity screening. By integrating species distribution modelling with trait-based screening, this study provides a framework for assessing climate resilience and guiding conservation and sustainable use strategies for medicinal plants under increasing thermal extremes.