This paper will consider a possible mechanism whereby electrical effects could come into play in the ascent of sap in tall trees. They concluded that: “…electrical effects on water lifting cannot be ignored a priori, even if they only trigger other processes that ultimately lead, via coupling, to bulk water flow.” -(ibid., p603) They also considered the possibility of electrical forces being involved, but without suggesting a mechanism whereby electrical forces could play a role. (2004) considered a number of alternative factors whereby a chemical gradient could be generated and thereby developed the theory of multi-force lifting. (2004) suggest an alternative based on the Plumb & Bridgman (1972) model, arguing that a chemical potential gradient could, in theory, balance the hydraulic pressure gradient. These considerations suggest that there may be additional mechanisms involved in the ascent of sap in tall trees, as Meinzer et al. 2004), well below the magnitude of the necessary tensions, implying that any slight disturbance, imperfection or pressure reduction due to constriction in the column of suspended sap would be expected to lead to immediate cavitation as air comes out of solution (Canny et al. The equilibrium vapour pressure of water is only +2 kPa at room temperature (Zimmermann U. If CTT is the only mechanism involved then the high tensions required for sap ascent in tall trees implies that the sap must be in a metastable state. the view formerly widely accepted, that the lumina of the conducting tissues during the time of active transpiration for the most part contain gas, if correct, would render our explanation untenable.” – (Dixon & Joly 1895, p571)Įven proponents of CTT acknowledge the presence of embolisms in the xylem (e.g. However it is worth remembering that Dixon & Joly themselves pointed out that: “…. Even those who argue for additional forces accept that pure water can sustain the tension necessary to support sap ascent in tall trees when the xylem is uncavitated. The arguments in favor of CTT are too well-known to need repeating here. Balling & Zimmermann 1990, Zimmermann U, Wagner & Schneider 2000 Zimmermann U. Other researchers suggest that alternative forces may be involved, and indeed may be necessary to explain both why cavitation in the xylem is not fatal to the flow and the various anomalous experimental results indicating that the predicted negative pressure gradients are often not found in living trees (e.g. Proponents of the widely-accepted Cohesion-Tension Theory (CTT), originally postulated separately by Dixon & Joly and Askenasy at the end of the nineteenth century, argue that CTT provides the only driving force needed to drive the ascent of sap in trees. Towards an Alternative Electrical Mechanism.Previous Electrical Measurements On Living Trees.“The Cohesion – Tension Theory has prevailed largely unmodified for over 100 years, a somewhat unusual situation for a scientific paradigm explaining a fundamental process.” (Meinzer, Clearwater & Goldstein 2001, p243) Article Outline The role of the natural vertical electric field of the Earth is discussed in relation to generation of the observed potentials in living trees and various corollaries of the inclusion of electrical forces are suggested. It is concluded that the electrical potential term in the water potential equation should be considered when investigating the range of possible forces involved in the ascent of sap.
Combining these results here leads to a possible mechanism whereby the known electrical potential differences in trees could assist in driving sap flow. More recently, it has been discovered that various mechanisms exist which can create large zones of charge separation in water. It has long been known that electrical potential differences exist in living trees but no viable mechanism has yet been proposed whereby these potential differences may help drive the flow in the xylem. The debate continues between proponents of the widely-accepted Cohesion-Tension Theory and those who consider that other forces, represented by the chemical potential equation, may also be involved in the ascent of sap. Received May 6th, 2013 Accepted October 19th, 2013 Published November 3rd, 2013 Available online November 10th, 2013 *Correspondence E-mail: Words: Ascent of Sap, Multi-force Theory, Electrical Forces The Ascent of Sap in Tall Trees: a Possible Role for Electrical Forces
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