Scaling of phloem structure and optimality of photoassimilate transport in conifer needles

The phloem vascular system facilitates transport of energy-rich sugar and signalling molecules in plants, thus permitting long-range communication within the organism and growth of non-photosynthesizing organs such as roots and fruits. The flow is driven by osmotic pressure, generated by differences in sugar concentration between distal parts of the plant. The phloem is an intricate distribution system, and many questions about its regulation and structural diversity remain unanswered. Here, we investigate the phloem structure in the simplest possible geometry: a linear leaf, found, for example, in the needles of conifer trees. We measure the phloem structure in four tree species representing a diverse set of habitats and needle sizes, from 1 (Picea omorika) to 35 cm (Pinus palustris). We show that the phloem shares common traits across these four species and find that the size of its conductive elements obeys a power law. We present a minimal model that accounts for these common traits...植物韧皮部（phloem）维管系统可介导能量富集的糖类与信号分子在植物体内的长距离运输，借此实现有机体内部的跨区域信号交流，并支撑根、果实等非光合器官的生长发育。该运输过程的动力源于渗透压，而渗透压由植物不同远端部位的糖类浓度差所产生。韧皮部是一套复杂的物质分配系统，目前学界关于其调控机制与结构多样性仍存在诸多未决问题。本研究针对最简单的几何构型——线性叶片（例如针叶树的针叶）中的韧皮部结构展开探究。我们选取了涵盖多样生境与针叶尺寸（范围从1厘米的塞尔维亚云杉（Picea omorika）至35厘米的长叶松（Pinus palustris））的4个树种，对其韧皮部结构进行测量。研究结果显示，这4个树种的韧皮部具有共通特征，且其传导组织的尺寸遵循幂律（power law）。我们构建了一套能够解释这些共通特征的极简模型……