How are leaf mechanical properties and water-use traits coordinated by vein traits? A case study in Fagaceae

Abstract

Leaf venation is a key trait determining leaf water relations, phloem transport and mechanical stability. However, since the contribution of leaf veins to mechanical properties has not been properly understood, our understanding of how leaf vein architecture adjusts these functions within leaves and across species as well as its mechanistic basis is also limited. Here, we tested the hypotheses that leaf water-use and mechanical properties would be more correlated with lower- and higher-order vein density (VLA), respectively. We studied eight Fagaceae species growing in temperate forests in Japan and differing in secondary vein patterns, leaf habits, size and natural habitat. We quantified two leaf water-use traits [lamina hydraulic conductance (Klamina) and carbon stable isotope ratio (δ13 C)], two mechanical properties [lamina tensile strength and tensile modulus of elasticity (E)] and nine traits relating to both vein architecture and leaf structure. Across species, primary and secondary vein density (VLA1·2) correlated positively to lamina strength and leaf structural traits (leaf area and leaf mass per area) but not to water use. In contrast, minor vein density (VLAmin) correlated to Klamina positively but not to mechanical properties and leaf structural traits. Water-use traits and mechanical properties were independent on both area and mass basis. Our results indicate that there is labour sharing in leaf functions among higher- and lower-order veins, that VLA1·2 influences leaf structural and mechanical properties and that VLAmin affects leaf water use and, thus, potentially the gas exchange capacity. These findings suggest that vein hierarchy is associated with functional differentiation among vein orders to support leaf functions. This system possesses different vein orders that are quantitatively and qualitatively different within leaves to carry out multiple functions. Therefore, the evolutionary or accumulative shift in vein hierarchy might explain the variation in leaf structure and functions.