MIT Mechanical Engineering, Cambridge, Massachusetts 02139, USA.
Department of Physics, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark.
Nat Plants. 2017 Mar 20;3:17032. doi: 10.1038/nplants.2017.32.
Vascular plants rely on differences in osmotic pressure to export sugars from regions of synthesis (mature leaves) to sugar sinks (roots, fruits). In this process, known as Münch pressure flow, the loading of sugars from photosynthetic cells to the export conduit (the phloem) is crucial, as it sets the pressure head necessary to power long-distance transport. Whereas most herbaceous plants use active mechanisms to increase phloem sugar concentration above that of the photosynthetic cells, in most tree species, for which transport distances are largest, loading seems, counterintuitively, to occur by means of passive symplastic diffusion from the mesophyll to the phloem. Here, we use a synthetic microfluidic model of a passive loader to explore the non-linear dynamics that arise during export and determine the ability of passive loading to drive long-distance transport. We first demonstrate that in our device, the phloem concentration is set by the balance between the resistances to diffusive loading from the source and convective export through the phloem. Convection-limited export corresponds to classical models of Münch transport, where the phloem concentration is close to that of the source; in contrast, diffusion-limited export leads to small phloem concentrations and weak scaling of flow rates with hydraulic resistance. We then show that the effective regime of convection-limited export is predominant in plants with large transport resistances and low xylem pressures. Moreover, hydrostatic pressures developed in our synthetic passive loader can reach botanically relevant values as high as 10 bars. We conclude that passive loading is sufficient to drive long-distance transport in large plants, and that trees are well suited to take full advantage of passive phloem loading strategies.
维管植物依靠渗透压的差异将糖从合成区域（成熟叶片）输出到糖汇（根、果实）。在这个过程中，被称为 Münch 压力流，从光合细胞向输出导管（韧皮部）装载糖是至关重要的，因为它设定了为远距离运输提供动力所需的压力头。虽然大多数草本植物使用主动机制将韧皮部糖浓度提高到光合细胞以上，但在大多数树木中，由于运输距离最大，装载似乎通过质外体扩散从叶肉到韧皮部被动发生，这与直觉相悖。在这里，我们使用一个被动加载器的合成微流控模型来探索在出口过程中出现的非线性动力学，并确定被动加载驱动远距离运输的能力。我们首先证明，在我们的设备中，韧皮部浓度由从源扩散加载的阻力和通过韧皮部的对流导出之间的平衡来设定。对流限制的导出对应于 Münch 运输的经典模型，其中韧皮部浓度接近源；相比之下，扩散限制的导出导致韧皮部浓度较小，流速与水力阻力的缩放较弱。然后，我们表明，在具有大运输阻力和低木质部压力的植物中，对流限制的导出的有效区域占主导地位。此外，在我们的合成被动加载器中产生的静水压力可以达到高达 10 巴的植物学相关值。我们得出的结论是，被动加载足以驱动大型植物的远距离运输，并且树木非常适合充分利用被动韧皮部加载策略。
