Petiole XLA (xylem to leaf area ratio) integrates hydraulic safety and efficiency across a diverse group of eucalypt leaves.

A theoretical trade-off between the efficiency and safety of water transport systems in plants is used to explain diverse ecological patterns, from tree size to community structure. Despite its pervasive influence, this theory has marginal empirical support. This may be partially due to obfuscation of associations by wide phylogenetic sampling or non-standard sampling between studies. To address this, we examine the coordination of structural and anatomical traits linked to hydraulic safety and efficiency in the leaves of an ecologically diverse group of eucalypts. We introduce a new trait for characterising leaf water transport function measured as the cross-sectional XA at the petiole divided by the downstream leaf area (XLA petiole ). Variation in XLA petiole revealed support for a safety-efficiency trade-off in eucalypt leaves. XLA petiole was negatively correlated with theoretical petiole xylem conductivity (K s_petiole ) and strongly negatively correlated with leaf cavitation vulnerability (Ψ50 leaf ). Species with lower Ψ50 leaf exhibited petiole xylem with narrower vessels and greater fibre wall area fractions. Our findings highlight XLA petiole as a novel integrative trait that provides insights into the evolution of leaf form and function in eucalypts and holds promise for wider use among diverse species.