Water transport, embolism recovery and water storage in trees

Abstract

The ability to maintain hydraulic continuity in the xylem is essential to supply leaves with the water that must be exchanged for carbon dioxide. The metastable nature of xylem sap causes this system to be inherently vulnerable to failure by rapid vaporization within the conduits. Much of the recent work on hydraulic architecture and cavitation has pursued the elusive mechanism behind apparent hydraulic recovery concurrent with tension in the bulk of the xylem, referred to as "novel refilling". An investigation into the dynamics of this behavior (Chapter 3) revealed two key artifacts that can produce the appearance of novel refilling when in fact no embolism (and therefore, no recovery) has occurred. A further implication of these artifacts is that plant xylem may be more robust against embolism than previously expected. In the absence of novel refilling, it becomes much harder to reconcile the extreme vulnerability reported for ring porous species. Studies of Robinia pseudoacacia (Chapter 4) address whether the artifacts illuminated in chapter 3 provide insight into the ongoing debate about the cavitation resistance of long-vesseled species and whether it is possible to accurately assess cavitation resistance in these species using the centrifuge method. Root pressure, as an alternative to novel refilling, provides plants with a means of reversing cavitation. Studies of Betula papyrifera (Chapter 5), however, show that recovery from embolism by root pressure is limited to early spring and point to an important role for water storage in fibers that minimizes xylem tensions and thus the risk of cavitation.