PREMISE The dimensions of phloem sieve elements have been shown to vary as a function of tree height, decreasing hydraulic resistance as the transport pathway lengthens. However, little is known about ontogenetic patterns of sieve element scaling. Here we examine within a single species (Quercus rubra) how decreases in hydraulic resistance with distance from the plant apex are mediated by overall plant size. METHODS We sampled and imaged phloem tissue at multiple heights along the main stem and in the live crown of four size classes of trees using fluorescence and scanning electron microscopy. Sieve element length and radius, the number of sieve areas per compound plate, pore number, and pore radius were used to calculate total hydraulic resistance at each sampling location. RESULTS Sieve element length varied with tree size, while sieve element radius, sieve pore radius, and the number of sieve areas per compound plate varied with sampling position. When data from all size classes were aggregated, all four variables followed a power-law trend with distance from the top of the tree. The net effect of these ontogenetic scalings was to make total hydraulic sieve tube resistance independent of tree height from 0.5 to over 20 m. CONCLUSIONS Sieve element development responded to two pieces of information, tree size and distance from the apex, in a manner that conserved total sieve tube resistance across size classes. A further differentiated response between the phloem in the live crown and in the main stem is also suggested.

中文翻译：

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红栎韧皮部筛管解剖结构的个体发育尺度和水力阻力随树高的变化

前提 韧皮部筛网元件的尺寸已被证明作为树木高度的函数而变化，随着运输路径的延长而降低水力阻力。然而，关于筛分元素标度的个体发育模式知之甚少。在这里，我们研究了单一物种（Quercus rubra）中水力阻力随距离植物顶点距离的降低是如何由整体植物大小介导的。方法我们使用荧光和扫描电子显微镜沿主茎和四种大小等级的树木的活树冠在多个高度对韧皮部组织进行采样和成像。筛元件长度和半径、每个复合板的筛面积数、孔数和孔半径用于计算每个采样位置的总水力阻力。结果 筛网元件长度随树的大小而变化，而筛元半径、筛孔半径和每个复合板的筛面积数随取样位置的变化而变化。当汇总所有大小类别的数据时，所有四个变量都遵循幂律趋势，距树顶的距离。这些个体发育尺度的净效应是使总液压筛管阻力与树高无关，从 0.5 到 20 m 以上。结论 筛管元件的开发响应两条信息，即树木的大小和距顶点的距离，其方式是在不同尺寸等级之间保持总筛管阻力。还提出了活冠韧皮部和主茎韧皮部之间的进一步分化反应。当汇总所有大小类别的数据时，所有四个变量都遵循幂律趋势，距树顶的距离。这些个体发育尺度的净效应是使总液压筛管阻力与树高无关，从 0.5 到 20 m 以上。结论 筛管元件的开发响应两条信息，即树木的大小和距顶点的距离，其方式是在不同尺寸等级之间保持总筛管阻力。还提出了活冠韧皮部和主茎韧皮部之间的进一步分化反应。当汇总所有大小类别的数据时，所有四个变量都遵循幂律趋势，距树顶的距离。这些个体发育尺度的净效应是使总液压筛管阻力与树高无关，从 0.5 到 20 m 以上。结论 筛管元件的开发响应两条信息，即树木的大小和距顶点的距离，其方式是在不同尺寸等级之间保持总筛管阻力。还提出了活冠韧皮部和主茎韧皮部之间的进一步分化反应。结论 筛管元件的开发响应两条信息，即树木的大小和距顶点的距离，其方式是在不同尺寸等级之间保持总筛管阻力。还提出了活冠韧皮部和主茎韧皮部之间的进一步分化反应。结论 筛管元件的开发响应两条信息，即树木的大小和距顶点的距离，其方式是在不同尺寸等级之间保持总筛管阻力。还提出了活冠韧皮部和主茎韧皮部之间的进一步分化反应。