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Plant function and evolutionary biology
RESEARCH ARTICLE

A thermodynamic analysis of the feasibility of water secretion into xylem vessels against a water potential gradient

Lars H. Wegner
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- Author Affiliations

Karlsruhe Institute of Technology, Institute for Pulsed Power and Microwave Technology (IHM), Campus North, Building 630, Hermann v. Helmholtz Platz 1, 76344 Eggenstein-Leopoldshafen, Germany. Email: lars.wegner@gmx.net

Functional Plant Biology 42(9) 828-835 https://doi.org/10.1071/FP15077
Submitted: 25 March 2015  Accepted: 20 June 2015   Published: 24 July 2015

Abstract

A series of recent publications has launched a debate on trans-membrane water secretion into root xylem vessels against a water potential gradient, energised by a cotransport with salts (e.g. KCl) that follow their chemical potential gradient. Cation–chloride–cotransporter -type transporters that function in this way in mammalian epithelia were detected in root stelar cells bordering on xylem vessels. Using literature data on barley (Hordeum vulgare L.) seedlings, one study confirmed that K+ and Cl gradients across stelar cell membranes favour salt efflux. Moreover, the energetic costs of putative water secretion into the xylem (required for maintaining ionic gradients) would amount to just 0.12% of the energy captured by photosynthetic C assimilation if transpirational water flow relied exclusively on this mechanism. Here, a detailed thermodynamic analysis of water secretion into xylem vessels is undertaken, including an approach that exploits its analogy to a desalinisation process. Water backflow due to the passive hydraulic conductivity of stelar cell membranes is also considered. By comparing free energy consumption by putative water secretion with (i) the free energy pool provided by root respiration and (ii) stelar ATPase activity, the feasibility of this mechanism is confirmed but is shown to depend critically on the plant’s energy status.

Additional keywords: aquaporin, bioenergetics, H+-ATPase, salt–water cotransport, respiration, root pressure.


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