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From Dixon To the Present
Curiously, research on the physics of sap ascent stopped more or less completely around the fifties. A determining factor was the lack of suitable methods to prove or disprove convincingly the statements of the CTT on the physical status of the xylem sap. Remember, at the time, neither the concept of water potential nor the experimental methods to measure the physical state of the xylem sap (micropsychrometer and pressure chamber techniques) existed (both concepts were introduced in the early sixties).
Another factor had a very important impact on the direction of future research: Van den Honert´s article “Water transport in plants as a catenary process,” published in 1948. This paper did not discuss the validity of the CTT (it just said that “the cohesion theory will be taken for granted”) but stressed a very simple yet powerful argument: since in well watered plants transpiration and absorption are almost equal, an Ohm’s law analogy can be used to determine the quantitative aspects of water flow from soil to leaves and atmosphere. Here the topic was not the physics of sap ascent; it was to determine which of the different resistances water passes on its way from the soil to the atmosphere is quantitatively most important. Van den Honert‘s concluded, “the master-process is always, under any circumstances, the transport in the gaseous part” (i.e., through the stomatal and boundary resistances). During the next thirty years, most studies on water transport dealt with the problem of evaluating gaseous and liquid resistances and their variations with climate and soil conditions. The resurrection of studies concerning the CTT started with the first paper (1966) of a series by John Milburn on the acoustic detection of cavitation in plants. After Milburn, Melvin Tyree, Martin Zimmermann, and their students were the pioneers who completely renewed and further developed the experimental approaches to the CTT. The present state and the most recent fates of this theory may be found in Web Essay 4.2.
Essay 2.2 The Cohesion–Tension Theory at Work
Pierre Cruiziat, PIAF-INRA-UBP, France, and Hanno Richter, University of Agricultural Sciences, Vienna
The cohesion-tension theory (CTT) has been advanced to explain the ascent of sap in plants, and especially, in trees. It relies on the physical properties of water, on mechanisms of liquid transport, and on the anatomical features of the xylem, the sap conducting system (see textbook Chapter 4). The CTT embodies the work of several dozen scientists from around the world over the course of a century (see Web Essay 4.1 for historical details). Even if some controversy and unsolved questions remain (see below), the CTT is nevertheless a very well founded explanation of the sap ascent in plants, and has been verified by a large number of experiments.
The CTT theory can be summarized in the following eight statements:
Winter embolism is best explained by a frost-thaw mechanism. When sap temperature drops a few degrees below 0°C, ice forms, creating air bubbles because gases dissolved in liquid sap are nearly insoluble in ice. It is assumed that during the thaw, when xylem tensions exceed a critical value, air bubbles are progressively released to the liquid phase, and air-water menisci are created. In contrast with summer embolism, winter embolism is highly dependent on the diameter of the conducting elements: the larger the conducting elements, the more likely the occurrence of embolism. Therefore, conifers bearing tracheids are much more resistant than ring-porous trees. This might explain the fact that conifers are much more abundant than broad-leaved trees in many cool areas.
Many experimental results provide strong support for the CTT. However, two arguments have been recently presented against this theory. The first comes from U. Zimmermann (Würzburg University, Germany) and co-workers. Zimmermann’s reasoning against the CTT is as follows:
Until a few years ago, it was not possible to measure with a pressure probe xylem tensions higher than 0.6 MPa (corresponding to potentials more negative than –0.6 MPa). The reason behind this limitation is that it is extremely difficult to puncture a vessel with a finely pointed capillary without disturbing the physical state of the water under tension within the vessel. In addition, the relatively large water reservoir in the probe and the materials used for its construction lead to spontaneous cavitations inside the instrument. Recent advances in the pressure probe technique have made it possible to measure pressures as low as –1 MPa. Although these values are not sufficiently negative to make this direct method a suitable alternative to the indirect ones, these findings indicate that the low values previously reported are due to technical limitations of the method. This indicates that Zimmermann’s argument against the CTT was never valid: it dealt with a technical limit of a method, not the validity of the CTT.
The second challenge to the CTT was presented by M. J. Canny and his group at the RSBC, Canberra, Australia. Canny proposed a different scheme for water movement, the compensating pressure theory. This theory claims that due to the fact that plant organs are rigid or have rigid outer layers, positive pressure may exist within them. The maximum pressure would be set by the osmotic pressure of the cell sap, and the osmotic pressure would be balanced by both wall stretching and the pressing of cells against each other in a confined space, the so called "tissue pressure." For each cell, the wall and tissue pressure together would make up the turgor pressure. This tissue pressure could be transmitted directly between different tissue regions; in particular, when xylem conduits pass through a tissue having tissue pressure, the positive pressure would extend into the xylem compartment. The tissue pressure would not affect flow rates in the xylem, which, according to Canny’s theory, are set by pumps and one-way valves in both roots and leaves. Since the tissue pressure does not change flux rate, it acts to elevate xylem pressures (Canny 1998, Canny 2001, Comstock 1999). The main objection to Canny’s suggested mechanism is that it is physically untenable and violates basic principles of thermodynamics (Tyree 1999, Comstock 1999). Canny’s opinion mainly comes from his cryo-scanning electron microscopy experiments. However, all present evidence suggests that the observed diurnal fluctuations in the filling state of frozen vessels are artifacts (Richter 2001, Tyree et al. 2002). Although Canny’s ideas are inconsistent with basic physical laws and lack experimental support, they have nevertheless stimulated a large number of critical experiments, raising interesting questions concerning refilling processes (Steudle 2001).
A recent synthesis of the main features of the CTT and the electrical analogy used for modeling water transport in the soil-plant-atmosphere continuum has led to a new approach to plant and tree water relations: the hydraulic architecture approach. This approach considers a plant, and especially a tree, as a hydraulic system. All hydraulic systems (dams, irrigation systems for crops or houses, the human blood vascular system) are composed of the same basic elements: a driving force, pipes, reservoirs and regulating systems. So described, the hydraulic architecture is a powerful tool to study the hydraulic characteristics of the conducting tissues under a whole range of natural conditions. Important questions subject to study with the hydraulic architecture approach include: What are the dynamics of cavitation? Does the microstructure of vessels (e.g., mechanical properties of pit membranes) affect water transfer and vulnerability to cavitation? What is the hydraulic specificity of the leaves? How important is the extravascular pathway, as compared to the vascular pathway? Can vulnerability curves provide a better understanding of the drought responses of trees?
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