A New Theory for the Ascent of Sap—Cohesion Supported by Tissue Pressure

Abstract

Recent work contradicting both the assumptions of the Cohesion Theory, and the tensions measured in the xylem sap by the pressure-chamber, is reviewed. Measurements with the xylem-pressure probe reveal pressures in vessels around 0 bar absolute, and no detectable gradients of pressure with tree height. Under high water stress, pressures down to -6 bar were found, but then cavitations occurred very readily. Also, measurements of the cavitation thresholds of water show an average threshold of about -2 bar. The uncertain foundations of the Cohesion Theory are recalled from the years before 1965. Soon after that date, Scholander's measurements with the pressure chamber were agreed to have confirmed the theory and the existence of high tensions in the xylem. Before 1965, many experiments over many years pointed to the conclusions now rediscovered, viz., no high tensions, and no gradients of tension. A resolution of these paradoxes is offered in the form of a new theory. This proposes that the driving force and the transmission of the force are the same as in the Cohesion Theory, but the operating pressure of the xylem is raised into a stable range by compensating tissue pressures pressing upon the tracheary elements. The tissue pressure does not propel the transpiration stream, which is still driven by evaporation, but protects the stream from cavitation. Evidence is presented for the existence of positive pressures in roots, wood, and leaves. It is shown that the anatomy of roots, wood, and monocotyledon and cryptogam vascular bundles is organized so that pressure is confined by mechanical barriers, and exerted upon the tracheary elements by the living cells of the phloem and the xylem parenchyma. The Compensating-Pressure Theory also explains, among other things, root pressure, the function of the endodermis, the structure of wood, the constant association of xylem and phloem, the absence of gas spaces in vascular tissue, the absence of a gravitational gradient in the xylem, bleeding from cut palm inflorescences, how insects are able to withdraw sap from the xylem, and the variable that is measured by the pressure chamber. This instrument measures the water potential, but this is the potential not of xylem in tension, but of the compensating pressure applied to the xylem. The requirements of the Theory are explained, and a number of predictions are made which are open to experimental testing.