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.树木内水分、糖分与营养物质的运输严格局限于特定的维管通路，因此不同器官之间可能存在相对的隔离性（即分区性（sectored））。已有研究证实，枝条内糖分与信号分子的转运存在显著的叶间分区通路，但此前尚未见水分运输相关分区性的报道。本研究针对6种温带落叶乔木的当年生枝条，测定了其叶间通路的水力分区性：其中3种为环孔材树种（Castanea dentata、Fraxinus americana、Quercus rubra），另外3种为散孔材树种（Acer saccharum、Betula papyrifera、Liriodendron tulipifera）。水力分区性的测定采用染色法与水力学法两种手段：在染色法中，我们切除叶片并将染料压入最基部的叶柄；针对每个叶柄，我们统计了其与该近基叶柄共有的维管束痕迹数量。对于其余枝条，我们测定了叶间通路的叶面积比导水率（kLL）。在6个树种中，有5个的分区性模式与叶序相符：纵向列生（orthostichous）叶对之间的维管束共享数与kLL值均显著高于非纵向列生的叶对。树种间的叶间分区性差异，通过非纵向列生与纵向列生叶对在共有维管束染色情况及kLL值上的比例差异来表征；针对6个树种，这两种分区性表征指标呈现极强的相关性（R²=0.94，P<0.001）。不同树种基于kLL的分区性差异可达8倍，且环孔材树种的分区性显著强于散孔材树种。叶间分区性的种间差异，对于枝条内叶片气体交换与水分关系的物种特异性协调具有重要意义，尤其在光照、水分与养分有效性发生波动时。