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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 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 the 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 the current evidence negates the conclu- sions drawn by Zimmermann from studies using the xylem pressure probe (XPP), but it is not yet clear in every instance why the XPP results disagree with those of other methods 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 both the XPP and traditional methods.

The Cohesion-Tension theory of sap ascent: current controversies