Root pressure and beyond: energetically uphill water transport into xylem vessels?

• Published in: Journal of experimental botany Vol. 65; no. 2; pp. 381 - 393
• Main Author: Wegner, Lars H
• Format: Journal Article
• Language: English
• Published: England Oxford University Press [etc.] 01.02.2014; Oxford University Press
• Subjects: Aquaporins; Aquaporins - metabolism; Arabidopsis; Biological Transport; Cell membranes; energy; mammals; metabolism; Models, Biological; OPINION PAPER; Parenchyma; physiology; Plant Roots; Plant Roots - physiology; plasma membrane; Pressure; rice; Root pressure; sap; Secretion; soil solution; solutes; Thermodynamics; transporters; trees; Vascular plants; Water; Water - metabolism; Water pressure; Water transportation; Xylem; Xylem - physiology; xylem vessels; cohesion–tension (CT) theory; reflection coefficient; water co-transport hypothesis; xylem refilling; CCC transporters; embolism repair; water ascent; Aquaporin; root pressure
• ISSN: 0022-0957; 1460-2431; 1460-2431
• DOI: 10.1093/jxb/ert391

The thermodynamics of root pressure remains an enigma up to the present day. Water is transported radially into xylem vessels, under some conditions even when the xylem sap is more dilute than the ambient medium (soil solution). It is suggested here that water secretion across the plasma membrane of xylem parenchyma cells is driven by a co-transport of water and solutes as previously shown for mammalian epithelia (Zeuthen T. 2010. Water-transporting proteins. Journal of Membrane Biology 234, 57–73.). This process could drive volume flow ‘energetically uphill’, against the free energy gradient of water. According to the model, solutes released by xylem parenchyma cells are subsequently retrieved from the sap at the expense of metabolic energy to maintain the concentration gradient that drives the water secretion. Transporters of the CCC type known to mediate water secretion in mammalian cells have also been found in Arabidopsis and in rice. The mechanism proposed here for root pressure could also explain refilling of embolized vessels. Moreover, it could contribute to long-distance water transport in trees when the cohesion–tension mechanism of water ascent fails. This is discussed with respect to the old and the more recent literature on these subjects.