Xylem Transport and the Negative Pressures Sustainable by Water

Experimental measurements of water's ability to sustain negative pressures are reviewed with special emphasis on the relevance of the results to xylem transport. Results vary over several orders of magnitude and depend on the conditions of testing. The greatest negative pressures are measured using heated, pressurized water or purified, degassed water. When conditions more closely approximating those found in biological systems are used, water is considerably weaker. From these measurements, one can predict the most likely range of negative pressures that can be sustained in the xylem. This range is between -0·1 and -0·6 MPa (absolute pressure). Negative pressures near -1 MPa are possible, but require stringent conditions. Negative pressures greater than -2 MPa are also possible, but unlikely, based on the experimental evidence.