Peters Winfried S, Knoblauch Michael
School of Biological Sciences, Washington State University, Pullman, WA, 99164, USA; Department of Biology, Purdue University Fort Wayne, Fort Wayne, IN, 46805, USA.
School of Biological Sciences, Washington State University, Pullman, WA, 99164, USA.
J Plant Physiol. 2022 May;272:153672. doi: 10.1016/j.jplph.2022.153672. Epub 2022 Mar 24.
Long-distance transport of photoassimilates in the phloem of vascular plants occurs as bulk flow in sieve tubes. These tubes are arrays of cells that lose nuclei, cytoskeleton, and some organelles when they differentiate into mature sieve elements. Symplasmic continuity is achieved by perforations that turn the cell walls between adjoining sieve elements into sieve plates. These structural features are interpreted as adaptations that reduce the resistance sieve tubes offer to cytoplasmic bulk flow. According to the common reading of Ernst Münch's pressure-flow theory, the driving forces for these flows are osmotically generated gradients of hydrostatic pressure along the sieve tubes. However, the significance of pressure gradients in the flow direction has also been questioned. Münch himself stated that no detectable pressure gradients existed between the linked osmotic cells that he used to demonstrate the validity of his ideas, and the earliest explanation of osmotically driven flows by Wilhelm Pfeffer, on which Münch based his theory, explicitly claimed the absence of pressure gradients. To resolve the apparent contradiction, we here reconstruct the history of the idea that osmotically driven transport processes in organisms necessarily require steps or gradients of hydrostatic pressure along the transport route. Our analysis leads us to conclude that some defects of overly simplifying interpretations of Münch's ideas (such as the sieve plate fallacy) could be avoided if our descriptions of his theory in textbooks and the scientific literature would follow the logics of the theory's earliest formulations more closely.
维管植物韧皮部中光合同化物的长距离运输以筛管中的集流形式发生。这些筛管是由细胞组成的阵列，当它们分化为成熟筛管分子时会失去细胞核、细胞骨架和一些细胞器。相邻筛管分子之间的细胞壁通过穿孔形成筛板，从而实现共质体连续性。这些结构特征被认为是适应性变化，可降低筛管对细胞质集流的阻力。根据对恩斯特·明希压力流理论的普遍理解，这些流动的驱动力是沿筛管的静水压的渗透产生梯度。然而，压力梯度在流动方向上的重要性也受到了质疑。明希本人指出，在他用来证明其观点有效性的相连渗透细胞之间不存在可检测到的压力梯度，而明希理论所基于的威廉·普费弗对渗透驱动流动的最早解释明确声称不存在压力梯度。为了解决这一明显的矛盾，我们在此重构了关于生物体中渗透驱动运输过程必然需要沿运输路线存在静水压步骤或梯度这一观点的历史。我们的分析使我们得出结论，如果我们在教科书和科学文献中对他的理论的描述更紧密地遵循该理论最早表述的逻辑，就可以避免对明希观点过度简化解释的一些缺陷（如筛板谬误）。
