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INTRODUCTION
In recent years, the validity of the cohesion-tension (CT) theory of the ascent of sap in plants has been questioned by Professor Ulrich Zimmermann and his colleagues at the University of Würzburg, Germany (4, 59, 143-145). Since the theory established over 100 years ago by Böhm (7) plays a key role in our understanding of the functioning of plants, the challenge has provoked a lively ongoing debate (e.g. 19a, 61, 67, 91, 97, 98, 113, 117, 128). New experimental approaches have been used to test the validity of the theory (19, 41, 73, 127, 129). Alternative or additional mechanisms have been proposed to explain the movement of water up to the top of the tallest trees (13, 14, 144). The topic has been reviewed recently by Tyree (117), who concluded that the evidence in favor of the cohesion-tension mechanism is strong, and by Tomos & Leigh (113), who discussed some aspects of pressure probe techniques involving the measurement of negative pressure. Unlike Tyree, Tomos & Leigh, perhaps still mindful of the intensity induced by the challenge to the cohesion-tension mechanism, avoid a clear judgment, namely a critical assessment of the use of pressure probes to measure tensions (negative pressures).
This review provides the physical background of the CT-mechanism, which necessitates a brief historical perspective, then discusses problems with the mechanism and recent evidence for and against it. The second part of the review considers the role of the cohesion-tension mechanism in the context of water acquisition by plant roots: how plants use both variable tension in the xylem and variable hydraulic properties of roots to regulate water uptake. Evidence that the uptake of certain solutes and their transport across the root may be affected by the CTmechanism...