Leaf Hydraulic Architecture and Stomatal Conductance: A Functional Perspective.

The structure of leaf vasculature viewed over a broad phylogenetic scale from lycophytes to eudicots correlates with stomatal conductance, providing the basis for the hypothesis that increasing vein density drove the evolution of high fluxes in angiosperms. Yet, the relationship between vascular geometry and gas fluxes breaks down at finer phylogenetic scales. In this Update, we derive a simple one-dimensional model suitable for mapping the effects of three-dimensional vascular architecture and transport capacity onto an effective length for transport through the mesophyll. This approach allows us to explore notions of optimality in the hydraulics of reticulately veined leaves, which differ from the two-dimensional case. We then consider limits to stomatal conductance that may derive from genomic constraints on cell size and the role of mechanical advantage. We conclude that more mechanistic modeling of transpiration could help resolve the sensitivity of stomatal, genome, and vein density relationships to the phylogenetic scale of the study.