Water ascent in plants: do ongoing controversies have a sound basis?

The cohesion-tension theory of the ascent of sap in plants is fundamental to the understanding of water movement in plants. According to the theory, water is pulled upwards by high tensions (low negative pressures) created in the xylem vessels and tracheids of higher plants by the evaporation of water vapour from leaves. However, much lower tensions (less negative pressures) have been found from direct measurements using a pressure probe. These do not appear to be compatible with the cohesion-tension theory. As a consequence, the validity of the cohesion-tension theory has been questioned and alternative mechanisms for sap ascent have been proposed. Recent experiments show that the conclusions drawn from the pressure probe work were premature. New direct measurements of xylem pressure support the cohesion-tension theory and the previous indirect measurements of xylem pressure.