Preprint Article Version 1 This version is not peer-reviewed

Long-Distance Water Transport of Land Plants Using the Thermodynamic Sorption Principle

Version 1 : Received: 23 February 2019 / Approved: 1 March 2019 / Online: 1 March 2019 (12:47:33 CET)

How to cite: Hahn, K. Long-Distance Water Transport of Land Plants Using the Thermodynamic Sorption Principle. Preprints 2019, 2019030011 (doi: 10.20944/preprints201903.0011.v1). Hahn, K. Long-Distance Water Transport of Land Plants Using the Thermodynamic Sorption Principle. Preprints 2019, 2019030011 (doi: 10.20944/preprints201903.0011.v1).

Abstract

In the case of vascular plants the process of water loss by leafs and water absorption by the root is well known. There is agreement on the passive nature of long-distance moisture movement in the dead cells of the xylem; however, controversy exists focusing on the long-distance water transport principle. Hales (1726) founded a view of bulk flow based on water suction after experiments with cut twigs. The previous doctrine of long-distance water transport within vessel elements and tracheid of the xylem of intact plants – the relevant cohesion theory in text books – was developed mainly by Boehm (1893), Renner (1911) and Dixon (1914) with plant artefacts.  Water movement according to this theory is based on an assumed hydrodynamic bulk fluid flow in xylem in continuous water columns (free of water vapour space), under tension, according to the law of Poiseuille (see e.g. Dixon 1914). Physically hydrodynamics is part of fluid mechanics, as a result Poiseuille’s law is usually valid only for hydrodynamic bulk flow in ideal capillaries (Sutera & Skalak, 1993). Besides the basic requirement for transport, according to cohesion theory, the existence of ideal capillaries is not compatible with either: “Because vessel elements and tracheid do not stand as ideal capillaries. …” (Bresinsky et al. 2008, translated from German). Unlike ideal capillaries, the walls of vessel elements and tracheid interact with the transported water.  These walls are able to function as a source or as a sink for the transported water because of interaction with the cell walls. With the interaction, vessel elements and tracheid, part of the xylem, can shrink and swell, unlike ideal capillaries. Because the xylem (in woody plants part of the wood) is inconsistent with the basic law of fluid flow, the equation of mass balance (Zimmermann et al. 2004) and cohesion theory are not strictly followed.Many plant physiologists view the cohesion theory as appropriate, however, this theory remains controver­sial, i.e. by Eisenhut (1988), Laschimke (1990) and Hahn (1993). Nultsch (1996) gives doubts referring to the present doctrine of plant water transport. Zimmermann et al. (2004) reject the cohesion theory and conclude: “... that the arguments of the proponents of the Cohesion Theory are completely misleading” (Zimmermann et al. 2004). Hence cohesion theory can be treated as inapplicable and the question arises: how does water transport in fact function?  In the following, it is gone into in more detail.  A sorption hypothesis of actual water transport, based on empirical fact, shall be addressed in this paper.

Subject Areas

Plant water transport, plant long-distance water transport, sorption hypothesis, cohesion theory, cohesion-tension theory

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