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

Recent work contradicting both the assumptions of the CohesionTheory, and the tensions measured in the xylem sap by the pressure-chamber,is reviewed. Measurements with the xylem-pressure probe revealpressures in vessels around 0 bar absolute, and no detectablegradients of pressure with tree height. Under high water stress,pressures down to -6 bar were found, but then cavitations occurredvery readily. Also, measurements of the cavitation thresholdsof water show an average threshold of about -2 bar. The uncertainfoundations of the Cohesion Theory are recalled from the yearsbefore 1965. Soon after that date, Scholander's measurementswith the pressure chamber were agreed to have confirmed thetheory and the existence of high tensions in the xylem. Before1965, many experiments over many years pointed to the conclusionsnow rediscovered, viz., no high tensions, and no gradients oftension. A resolution of these paradoxes is offered in the formof a new theory. This proposes that the driving force and thetransmission of the force are the same as in the Cohesion Theory,but the operating pressure of the xylem is raised into a stablerange by compensating tissue pressures pressing upon the trachearyelements. The tissue pressure does not propel the transpirationstream, which is still driven by evaporation, but protects thestream from cavitation. Evidence is presented for the existenceof positive pressures in roots, wood, and leaves. It is shownthat the anatomy of roots, wood, and monocotyledon and cryptogamvascular bundles is organized so that pressure is confined bymechanical barriers, and exerted upon the tracheary elementsby the living cells of the phloem and the xylem parenchyma.The Compensating-Pressure Theory also explains, among otherthings, root pressure, the function of the endodermis, the structureof wood, the constant association of xylem and phloem, the absenceof gas spaces in vascular tissue, the absence of a gravitationalgradient in the xylem, bleeding from cut palm inflorescences,how insects are able to withdraw sap from the xylem, and thevariable that is measured by the pressure chamber. This instrumentmeasures the water potential, but this is the potential notof xylem in tension, but of the compensating pressure appliedto the xylem. The requirements of the Theory are explained,and a number of predictions are made which are open to experimentaltesting.Copyright 1995, 1999 Academic Press Ascent of sap, cavitation, cohesion theory, endodermis, pressure chamber, root pressure, stem pressure, tissue pressure, transpiration, water potential, wood anatomy, xylem pressure