The Cohesion-Tension Theory of Sap Ascent

Abstract

The cohesion-tension theory is also sometimes called the cohesion theory. It is a rather old theory dating back to the late nineteenth century, which attempts to explain the principal forces that govern the 'ascent of sap' from the soil to the leaves in plants. It was a very controversial theory when first proposed (Bi:ihm 1893; Dixon 1914, pp. 142-154) and it has been the focus of controversy about every 20 years since it was postulated. At the time of writing, there has been more controversy over the cohesion theory than at any other time, excluding the initial controversy when it was originally proposed. The theory, at the very least, is incomplete in the sense that it describes a 'universal' motive force for water movement in plants. This should not surprise us because nineteenth century science had an incomplete understanding of what makes water move in plants because the science of thermodynamics was not mature enough to provide plant physiologists with the necessary insights. While the cohesion-tension theory is not perfect and has a few unsolved problems, I personally feel much of the current debate has proved to have little merit, as will become evident in this and subsequent chapters. The principal motive force driving long-distance sap ascent in plants is water pressure and pressure plays a major role in the cohesion theory. The controversial aspect of the cohesion-tension theory is the 'improbable' concept of water under tension, which is equivalent to negative pressure in a fluid. Tension is easy to understand in a solid object, e.g., a rope used to lift a weight. While a weight is lifted with the aid of a rope, the rope is under tension; the fibers in the rope are being pulled apart by two opposing forces-the force of gravity acting on the weight and the counter force at the other end of the rope where the person or machine is pulling on it. The cohesion theory proposes that water can be under tension in the special circumstances that exist in plants. The tension is supposed to be generated in the leaves at the liquid-air interface where water evaporates from leaves. However, before we get into the question of how the tension is generated , how much tension is needed to move water up a large tree, and how water can be stable under tension, we must review the nuances of the motive force for water movements in plants. This will allow us to view the cohesion theory in a more complete and modern context. Most people are willing to accept the hypothesis that water moves in plants passively, which simply means that plants do not have metabolic water pumps. The equivalent organ in animals for a metabolic pump would be a heart. When water and solutes can move equally freely, e.g., in a xylem vessel, then passive movement is driven mostly by pressure gradient. When solute movement is M. T. Tyree et al., Xylem Structure and the Ascent of Sap