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

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