Tolerance for heat stress in Lemna minor reflects both local adaptation and acclimation over time

Assessing intraspecific variation in thermal stress tolerance is key to predicting plant responses and long-term persistence under climate change, yet its underlying sources and temporal dynamics remain poorly understood. Using a common garden experiment with four ecologically-relevant temperatures, we evaluated the sources and temporal dynamics of variation in temperature stress tolerance of 18 Lemna minor clonal lines from contrasting climates. Our results showed that past adaptation, physiological acclimation, and within-line variation jointly contributed to variation in performance. The study provides the first evidence of adaptive genetic differentiation in heat stress tolerance in this ecologically-widespread freshwater species, with lines from warmer regions showing higher growth under heat stress. However, these differences were transient and diminished under prolonged exposure. Experimental lines also showed acclimation over time, but these responses were strongly temperature-dependent and occurred only under sub-optimal conditions. Additionally, replicates from some lines exhibited divergent performance trajectories under sustained heat stress, suggesting the emergence of novel phenotypic variation, potentially mediated by epigenetic mechanisms. These results show that heat stress tolerance in L. minor arises from multiple interacting sources and is dynamically shaped by both selective history and immediate exposure time, suggesting a more nuanced, multi-layer understanding of variation in heat stress tolerance.