Perspectives on the Biophysics of Xylem Transport

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This chapter provides a brief discussion on cohesion-tension theory. The chapter also focuses on factors that affect hydraulic resistance in plants. It addresses how sap is extruded into the empty tracheary element even though the free energy gradient from soil to xylem seems often to be in the wrong direction, how the element is hydraulically isolated during refilling to avoid its new contents being sucked away in the transpiration stream, and how a refilled vessel ultimately reestablishes hydraulic contact with neighboring conduits in such fashion as to become once again useful in water transport. For xylem sap to flow through the plant, the negative hydrostatic pressure gradient must be large enough to overcome the "resistive" force of sap viscosity and the force of gravity. Because the rate of xylem sap flow through plants is relatively slow, the laminar flow pattern within an individual vessel consists of layers of sap that flow past each other in a sheetlike fashion, with the result that neither eddies nor vortices occur and turbulence is absent. However, in both laminar and turbulent flow, viscosity leads to internal friction, which produces energy dissipation. The cohesion-tension theory still stands as the hegemonic model for understanding long-distance sap transport in plants, but a controversy related to it propelled the development of new ideas and possible alternative paradigms to replace the postulates of cohesion-tension theory. © 2005 Elsevier Inc. All rights reserved.

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Vascular Transport in Plants

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