Let's not forget the critical role of surface tension in xylem water relations.

The widely supported cohesion–tension theory of water transport explains the importance of a continuous water column and the mechanism of long-distance ascent of sap in plants (Dixon 1914, Tyree 2003, Angeles et al. 2004). The evaporation of water from the surfaces of mesophyll cells causes the air–water interface to retreat into the cellulose matrix of the plant cell wall because the cohesion forces between water molecules are stronger than their attraction to air. As a result, the interface between the gas and liquid phases places the mass of water under negative pressure (tension). This pulling force is then transmitted to soil water via a continuous water column since the strong hydrogen bonding of the water molecules also allows water to stay liquid under tension (Oertli 1971). Related to these cohesive forces is surface tension, which characterizes how difficult it is to stretch the surface of a liquid. Most laboratory and field studies dealing with xylem cavitation and embolism repair assume that surface tension is equal to that of pure water and constant within and between species. Although surface tension is a crucial parameter in xylem water movement, few studies have tested whether this parameter differs from that of pure water (Bolton and Koutsianitis 1980). In this issue, the study by Christensen-Dalsgaard et al. (2011) looked at the instantaneous surface tension of xylem sap extracted from branches of three tree species and its change over time. Using the pendant-drop method, they showed that in all three species studied, the instantaneous sap surface tension was indeed equal to that of pure water. However, in one species, Populus tremuloides, surface tension decreased by 15% after half an hour, which was related to the formation of surfactants caused by amphiphilic molecules present in the xylem sap. Moreover, this lower surface tension was more pronounced in terminal branches, with a reduction of 25% compared with pure water. These results have several implications for water movement in plants. The first is related to winter embolism because the radius above which an air bubble will expand rather than disappear upon thawing is proportional to the surface tension of the xylem sap. The breakdown of water columns in xylem conduits following a frost–thaw event is due to the expansion of air bubbles formed during sap freezing. If the difference in gas pressure in the bubble and the sap pressure is less than the capillary pressure originating from the surface tension, then the gas will dissolve. If upon thawing the sap is still under lower surface tension than that of pure water, then the bubbles will dissolve at less negative xylem pressure. The direct consequence of lower xylem sap surface tension will therefore be to limit the reduction in xylem conductivity of trees due to winter embolism. Second, it indicates that in some species this decrease in the surface tension of xylem sap will increase the vulnerability of xylem to embolism (Cochard et al. 2009). There is considerable evidence that rather than conduit diameter, it is the pore diameters in the intervessel pit membranes that determine a conduit’s vulnerability to water stress-induced embolism. If cavitation occurs, or if a conduit is damaged (through herbivory for example), the air bubble does not expand to a neighboring conduit because of the surface tension effect at the pore of the pit membranes (Bailey 1916). The radius of curvature (r) needed to sustain a difference in pressure (P) at which a pit fails as a barrier and allows gas to enter the vascular system depends directly on the liquid’s surface tension Commentary