Leaf hydraulic vulnerability is related to conduit dimensions and drought resistance across a diverse range of woody angiosperms

Summary: Hydraulic dysfunction in leaves determines key aspects of whole-plant responses to water stress; however, our understanding of the physiology of hydraulic dysfunction and its relationships to leaf structure and ecological strategy remains incomplete. Here, we studied a morphologically and ecologically diverse sample of angiosperms to test whether the water potential inducing a 50% loss in leaf hydraulic conductance (P50leaf) is predicted by properties of leaf xylem relating to water tension-induced conduit collapse. We also assessed the relationships between P50leaf and other traits considered to reflect drought resistance and ecological strategy. Across species, P50leaf was strongly correlated with a theoretical predictor of vulnerability to cell collapse in minor veins (the cubed ratio of the conduit wall thickness to the conduit lumen breadth). P50leaf was also correlated with mesophyll traits known to be related to drought resistance, but unrelated to traits associated with carbon economy. Our data indicate a link between the structural mechanics of leaf xylem and hydraulic function under water stress. Although it is possible that collapse may contribute directly to dysfunction, this relationship may also be a secondary product of vascular economics, suggesting that leaf xylem is dimensioned to avoid wall collapse.