Limitations in the Hydraulic Pathway: Effects of Xylem Embolisms on Sap Velocity and Flow

Sap flow in plants takes place in the xylem, a hydraulic system that is usually under negative pressure and in which gas and liquid phases are separated by nanoporous, fibrous pit membranes. It has long been known that this system is at risk of drawing gas nanobubbles through these membranes into the xylem sap, a process referred to as “air seeding”. These bubbles then can cavitate and create embolisms. Embolized vessels and tracheids block the hydraulic pathway, reduce hydraulic conductivity, and thereby potentially reduce sap flow. Under drought stress, the number of sap-filled conduits often steadily declines with increasingly negative xylem water potential. It was long thought that removal of embolisms would be physically impossible while the system is transporting water under negative pressure. However, recent research has provided abundant evidence for seasonal and/or diurnal formation and removal of xylem embolisms in many plant species. The number of functioning conduits as well as wood water and gas content can fluctuate over a growing season and often over the course of a day. We review evidence for such changes obtained through hydraulic measurements, cryo-scanning electron microscopy, and high-resolution computed tomography. Loss and gain of functional conduits over time effectively translates into changes in the xylem conducting area. In sap flow research, conducting area is almost universally assumed to be constant over time, but this assumption turns out to be invalid for plants that form and repair embolisms. Temporal changes in the water content of other wood cells also affect the thermal conductivity of wood and therefore sap flow measurements based on heat-fluxes. Measurements of active conducting area and wood water content concurrently with sap flow measurements may be needed for accurate determination of sap flux density and volumetric flow. INTRODUCTION Sap flow takes place in the xylem, the hydraulic system of vascular plants. Capillary tension in cell walls of leaves creates negative pressure in the xylem, which in turn drives the flow of sap from roots to leaves. Dixon and Joly (1895) were the first to note that the xylem was well protected from intrusion of air bubbles into the sap, and they suggested that water-soaked cell walls are essentially air-tight. In fact, any air that gets into the xylem has to overcome the high surface tension of air-water interfaces in the nanopores of cell walls and fibrous pit membranes that connect xylem conduits, including vessels and tracheids (Tyree and Zimmermann, 2002). It typically takes very large pressure differentials to force air into xylem, a process known as air-seeding. Although a few plants appear to have evolved xylem pit membranes that are essentially air-tight under natural conditions and can withstand pressure differentials of 8 MPa or more (Maherali et al., 2004), most plants experience at least some entry of nanometer-sized air bubbles (nanobubbles) into the xylem when xylem pressure falls below a critical value, known as the “air seeding” pressure (Tyree and Zimmermann, 2002). Under declining pressure, these nanobubbles can cavitate and quickly expand to fill the whole conduit and form an embolism, thereby obstructing sap flow. Embolism formation was long thought to be a catastrophic event in xylem, because refilling of conduits while the xylem was transporting sap under negative Proc. 9 International Workshop on Sap Flow