Differentiation for drought strategy but conserved plasticity to heat and drought in locally adapted populations of

Background and Aims: Adaptive plasticity may be crucial for plants to survive rapid environmental changes long enough for evolutionary adaptation to occur. Heat and drought are both abiotic stresses projected to increase globally and their individual impact on plant growth and function is well-characterized. However, to understand potential responses to climate change, we must manipulate multiple stressors simultaneously. Methods: We measured heat- and drought-related traits as well as fitness in two locally adapted populations of Key Results: Population differentiation in both treatments suggests that the Swedish population avoids drought by investing in stress-tolerant leaves while the Italian population escapes drought by flowering early. Despite these differences, there is little evidence for genetic differentiation of plasticity; when experiencing heat and drought, both populations shift their traits in the direction expected to avoid drought. Further, both populations have greatly decreased fitness in heat and drought. Conclusions: These results highlight that locally adapted populations with genetic differentiation for traits within a single environment can respond to the same stressors with plasticity in the same direction. Further, the combination of heat and drought will be extremely damaging to plant populations.