A New Evidence for the Cohesion-tension Theory

Although the cohesion-tension theory has been well established for more than a century, some probelms have stood in the way of its complete acceptance because its corollary is difficult to prove: that large negative pressures exist in xylem conduits. Especially recent xylem pressure-probe and Z-tube experiments suggest that cavitation limits xylem pressures to above -0.5 MPa, which may necessitate modification of the cohesion-tension theory of xylem transport. Now another two research groups used a similar modified Z-tube method to determine negative pressure causing cavitation in intact xylem conduits. This show s that xylem conduits of trees remained water-filled and conductive to species-specific pressures ranging from -1.2 to below -3.5 MPa, supporting the cohesion-tension theory as the primary mechanism for water transport in trees.