THE COHESION-TENSION THEORY OF SAP ASCENT - CURRENT CONTROVERSIES

In recent years, the Cohesion-Tension (C-T) theory of sap ascent in pl ants has come under question because of work published by Professor Ul rich Zimmermann and colleagues at the University of Wurzburg, Germany. The purpose of this review is to (1) state the essential and testable elements of the C-T theory, (2) summarize the negative evidence for t he C-T theory, and (3) review critically the positive evidence for the C-T theory and the evidence that the Scholander-Hammel pressure bomb measures xylem pressure potential (P-x) correctly, because much of the evidence for the C-T theory depends on pressure bomb data. Much of th e current evidence negates the conclusions drawn by Zimmermann from st udies using the xylem pressure probe (XPP), but it is not yet clear in every instance why the XPP results disagree with those of other metho ds for estimating xylem pressure. There is no reason to reject the XPP as a useful new tool for studying xylem tensions in the range of 0 to -0.6 MPa. Additional research is needed to test the C-T theory with b oth the XPP and traditional methods.