Breaking the rules: the hydraulic basis for trait climate relationships in Arabidopsis

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Leila Fletcher and Camila Dias Barros Medeiros, Ecology and Evolutionary Biology, University of California, Los Angeles, Los Angeles, CA, Christine Scoffoni, Department of Biological Sciences, California State University, Los Angeles, CA, Kevin Sartori, Department of Plant Biology, Swedish University of Agricultural Sciences, Uppsala, Sweden, Francois Vasseur and Cyrille Violle, Centre d'Ecologie Fonctionnelle et Evolutive, CNRS, Montpellier, France, Colin Farrell and Matteo Pellegrini, Molecular, Cell, & Developmental Biology, University of California, Los Angeles, Los Angeles, CA, Lawren Sack, Department of Ecology & Evolutionary Biology, University of California Los Angeles, Los Angeles, CA

Background/Question/Methods
Climate change and global droughts are impacting the distributions of species and genotypes. Leaf hydraulic traits are major determinants of drought tolerance and the maximum rate of gas exchange. However, while the relationships between these traits and climate have been tested across diverse and closely-related species, there have been no rigorous tests across populations of a given species. Model species Arabidopsis thaliana provides an excellent platform to elucidate potential mechanisms and trade-offs in drought responses. We tested variation across a broad range of 156 Arabidopsis ecotypes with a multi-continental distribution to investigate the leaf osmotic potential at full turgor (πo), a main determinant of the leaf wilting point, a key predictor of drought tolerance as a measure of the ability of plants to maintain photosynthetic function as soil moisture declines and its relationship with climate. Additionally, for 12 diverse genotypes grown in a common garden we tested the association of πo with gas exchange and hydraulic traits, including the leaf hydraulic conductance (Kleaf), operating stomatal conductance (gop), and light-saturated photosynthetic rate (Amax), traits that are typically tightly correlated across diverse species.
Results/Conclusions
We found that Arabidopsis considered across a very wide range breaks the rules for hydraulic adaptation to climate. Contrary to previous studies for diverse species, and studies of smaller sets of Arabdiopsis ecotypes across diverse genotypes, πo was weakly related to climate, but strongly related to flowering time, consistent with shorter-lived ecotypes avoiding cold and drought and adapting to complete their growth quickly in favorable conditions. This result was supported by coordination of high πo with high gop and Amax. In contrast with trends shown across diverse species, Amax was decoupled from Kleaf, indicating that hydraulic supply is not limiting to the growth of genotypes that complete their lifecycles only in times of water availability. These results highlight the contrasting physiological adaptation of stress-avoidant annual species, the departure of Arabidopsis from typical paradigms of physiological optimality theory, and the importance of investigating physiological relationships at multiple scales, including within a species.