Hydraulic Pathways in Leaves of Temperate Trees at Harvard Forest 2002

The transport of water, sugar and nutrients in trees is restricted to specific vascular pathways, and thus organs may be relatively isolated from one another (=sectored). Strongly sectored leaf-to-leaf pathways have been shown for the transport of sugar and signal molecules within a shoot, but not previously for water transport. The hydraulic sectoriality of leaf-to-leaf pathways was determined for current year shoots of six temperate deciduous tree species (three ring-porous: Castanea dentata, Fraxinus americana and Quercus rubra, and three diffuse-porous: Acer saccharum, Betula papyrifera and Liriodendron tulipifera). Hydraulic sectoriality was determined using dye staining and a hydraulic method. In the dye method, leaf blades were removed, and dye was forced into the most proximal petiole. For each petiole we counted the vascular traces shared with the proximal petiole. For other shoots, measurements were made of the leaf-area specific hydraulic conductivity for leaf-to-leaf pathways (kLL). In five of six species patterns of sectoriality reflected phyllotaxy; both the sharing of vascular bundles between leaves and kLL were higher for orthostichous than non-orthostichous leaf pairs. Species-differences in leaf-to-leaf sectoriality were determined as the proportional differences between non-orthostichous vs. orthostichous leaf pairs in their staining of shared vascular bundles and in their kLL; for the six species these two indices of sectoriality were strongly correlated (R2 = 0.94; P less than 0.001). Species varied 8-fold in their kLL-based sectoriality, and ring-porous species were more sectored than diffuse-porous species. Differential leaf-to-leaf sectoriality has implications for species-specific coordination of leaf gas exchange and water relations within a branch, especially during fluctuations in irradiance, water and nutrient availability.