Differentiation for drought strategy but conserved plasticity to heat and drought in locally adapted populations of <i>Arabidopsis thaliana</i>

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. Drought response, in particular, is categorized into three drought survival strategies: escape, avoidance, and tolerance. However, to understand potential responses to climate change, we must manipulate multiple stressors simultaneously. We do not know what role plasticity to the combination of heat and drought plays in drought strategy or how plasticity will affect adaptation to projected future climates. METHODS: We measured heat- and drought-related traits as well as fitness in two locally adapted populations of Arabidopsis thaliana from Rödåsen, Sweden and Castelnuovo di Porto, Italy. We used chamber common gardens that simulate the current fall and spring climate in Sweden and a hotter and drier climate to identify population differentiation for trait means, trait plasticity, and fitness. 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 increased heat and drought will be extremely damaging to plant populations.