The Role of Cellulose Fibers inGnetum gnemonLeaf Hydraulics

Abstract

Premise of research. Leaves of Gnetum are strikingly similar to angiosperm leaves with their reticulate venation in a broad elliptic lamina. Despite this superficial resemblance, their leaf vein density—a parameter that has been shown to be a key determinant of hydraulic and photosynthetic capacity—is well below what might be expected for an angiosperm tree in humid tropical forests, and Gnetum vein placement within the leaf tissue is similarly problematic given its relatively high stomatal conductance.Methodology. We used a combination of basic physiology with scanning electron microscopy and light microscopy to study the distribution and properties of G. gnemon leaf fibers to determine whether they play a direct role in water transport and leaf hydration.Pivotal results. We show that cellulosic fibers penetrating the leaf mesophyll actively participate in water distribution in G. gnemon leaves, providing a hydraulic supplement to vein-based water distribution.Conclusions. Presence of this alternative water supply path explains high stomatal conductance and stomatal pore indices that are inconsistent with their low vein densities. These fibers act in a way that is analogous to the transfusion tissue of some cycads and conifers, although the mechanism involved is unique since conductance is via thick hydrophilic cell walls rather than via cell lumens, as in lignified tracheids. Thus, the superficial convergence of Gnetum leaves with overall angiosperm morphology obscures the fact that, functionally, they are still very much gymnospermous.