Review article. The cohesion-tension theory of sap ascent: current controversies

believe to be the strongest, quantitative evidence available for the C–T theory. In recent years, the Cohesion–Tension (C–T) theory of sap ascent in plants has come under question because of work published by Professor Ulrich Zimmermann Essential elements of the C–T theory and colleagues at the University of Würzburg, The C–T theory was proposed 103 years ago by Dixon Germany. The purpose of this review is to (1) state the and Joly (1894), and some aspects of the C–T theory essential and testable elements of the C–T theory, (2) were put on a quantitative basis by van den Honert summarize the negative evidence for the C–T theory, (1948) with the introduction of the Ohm’s law analogue and (3) review critically the positive evidence for the of sap flow in the soil–plant–atmosphere continuum. C–T theory and the evidence that the Scholander– According to the C–T theory, water ascends plants in Hammel pressure bomb measures xylem pressure a metastable state under tension, i.e. with xylem pressure potential (P x ) correctly, because much of the evidence x more negative than that of the vapour pressure of for the C–T theory depends on pressure bomb data. water. The driving force is generated by surface tension Much of the current evidence negates the concluat the evaporating surfaces of the leaf. The tension is sions drawn by Zimmermann from studies using the transmitted through a continuous water column from the xylem pressure probe (XPP), but it is not yet clear in leaves to the root apices and throughout all parts of the every instance why the XPP results disagree with apoplast in every organ of the plant. Evaporation occurs those of other methods for estimating xylem pressure. predominantly from the cell walls of the substomatal There is no reason to reject the XPP as a useful new chambers due to the much lower water potential of the tool for studying xylem tensions in the range of 0 to water vapour in air. The evaporation creates a curvature −0.6 MPa. Additional research is needed to test the in the water menisci of apoplastic water within the C–T theory with both the XPP and traditional methods. cellulosic microfibril pores of cell walls. Surface tension forces lower x in the liquid directly behind the menisci