Phloem Transport: The Surface Flow Hypothesis

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Abstract
Various authors have suggested that phloem contains structural components that provide interfaces at which surface movement could be the mechanism of transport along phloem. Although surface flow at a liquid/gas interface or a liquid/liquid interface could be a sufficiently rapid process provided both media can flow with the interface, it is demonstrated that a liquid/solid interface imposes an excessive viscous resistance. A calculated velocity for surface flow along stationary fibrils of P-protein under a gradient of surface energy is much lower than observed translocation velocities. Moving fibrils would carry little sucrose by the calculated absorption excess of the interface layer; moreover free fibrils would tend to move towards the region of higher sucrose concentration. Similar arguments discount the plasmalemma and the tonoplast as suitable structures. A predicted value of attraction to the endoplasmic reticulum would not provide enough energy for an adequate driving force. Surface diffusion at a liquid/liquid or liquid/solid interface is not a sufficiently rapid process. The sole possibility speculatively requires that there be a high affinity between sucrose, or some other molecule, and the surface of the endoplasmic reticulum, and further that it be free to flow from element to element with a continuous rate of supply of membrane components at a source, of their removal at a sink, and of their recirculation in some way. Since such a system seems implausible, it is concluded that neither surface flow nor surface diffusion can be the mechanism for transport along phloem.

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