Peter Melcher (Biology) presented, "Embolism Refilling under negative water potentials – Fact or Fiction?" at the University of Vermont, James Marvin Memorial Seminar Series on 10/23/14. See link for details. http://www.uvm.edu/~plantbio/marvin.php
Abstract:
Animals use a positive pressure pump called a heart to push fluids around their bodies. Plants, on the other hand, have evolved a negative pressure pump system that relies on the evaporative forces of the atmosphere to pull water through their bodies. This pulling force is needed to overcome the forces of hydraulic resistance and gravity as the water moves through the soil-plant-atmosphere continuum. Rather large negative hydrostatic pressures are generated on the continuous water columns located in the xylem of plants during times of rapid transpiration and/or drought. Because water under negative hydrostatic pressures is metastable, air bubbles can form and break the continuity of the water columns leading to water deficits experienced at the leaf level. Depending on the severity of the spread of embolism, this can cause stomatal closure, leaf wilting, and if prolonged, eventual plant death. Studies have shown that some plants have the ability to repair these embolisms after they form, and in some causes this has been reported to occur even during times when the plant water potentials are negative. This seems to break thermodynamic laws and thus make it difficult to reconcile. The mechanisms that allow plants to refill embolisms while under tension have been difficult to test and has been a hot topic in the field since the first observations of embolism refilling were made 15 years ago. In this talk, I will present data measured at the whole plant level on trees using hydraulic methods and Magnetic Resonance Imaging to show embolism refilling while all surrounding water potentials were negative. I will also provide potential mechanisms that have been proposed in the literature to describe how this refilling process might work.
https://www.ithaca.edu/intercom/article.php/20141027144951975