The surface tension of xylem sap has been traditionally assumed to be close to that of the pure water because decreasing surface tension is thought to increase vulnerability to air seeding and embolism. However, xylem sap contains insoluble lipid-based surfactants, which also coat vessel and pit membrane surfaces, where gas bubbles can enter xylem under negative pressure in the process known as air seeding. Because of the insolubility of amphiphilic lipids, the surface tension influencing air seeding in pit pores is not the equilibrium surface tension of extracted bulk sap but the local surface tension at gas-liquid interfaces, which depends dynamically on the local concentration of lipids per surface area. To estimate the dynamic surface tension in lipid layers that line surfaces in the xylem apoplast, we studied the time-dependent and surface area-regulated surface tensions of apoplastic lipids extracted from xylem sap of four woody angiosperm plants using constrained drop surfactometry. Xylem lipids were found to demonstrate potent surface activity, with surface tensions reaching an equilibrium at ~25 mN m-1 and varying between a minimum of 19 mN m-1 and a maximum of 68 mN m-1 when changing the surface area between 50 and 160% around the equilibrium surface area. It is concluded that xylem lipid films in natural conditions most likely range from nonequilibrium metastable conditions of a supersaturated compression state to an undersaturated expansion state, depending on the local surface areas of gas-liquid interfaces. Together with findings that maximum pore constrictions in angiosperm pit membranes are much smaller than previously assumed, low dynamic surface tension in xylem turns out to be entirely compatible with the cohesion-tension and air-seeding theories, as well as with the existence of lipid-coated nanobubbles in xylem sap, and with the range of vulnerabilities to embolism observed in plants.

中文翻译：

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木质部树液脂质的动态表面张力。

传统上认为木质部树液的表面张力接近纯水的表面张力，因为认为降低表面张力会增加对空气播种和栓塞的脆弱性。然而，木质部树液含有不溶性的基于脂质的表面活性剂，该表面活性剂也覆盖了容器和凹坑的膜表面，在这种过程中，气泡可以在负压下以称为空气接种的过程进入木质部。由于两亲脂质的不溶性，影响坑穴中空气播种的表面张力不是提取的大块树液的平衡表面张力，而是气液界面处的局部表面张力，这动态取决于每个表面积上脂质的局部浓度。要估算在木质部质外体表面排列的脂质层中的动态表面张力，我们研究了使用约束滴度表面测量法从四种木本被子植物木质部汁液中提取的质外脂质的时间依赖性和表面积调节的表面张力。发现木质部脂质表现出有效的表面活性，当表面张力在50至50 mN m-1之间变化时，表面张力在〜25 mN m-1处达到平衡，并且在最小19 mN m-1和最大68 mN m-1之间变化。平衡表面积的160％。结论是，取决于气液界面的局部表面积，在自然条件下木质部脂质膜最有可能从过饱和压缩状态的非平衡亚稳定状态到不饱和膨胀状态。连同发现被子植物凹坑膜中的最大孔收缩比以前假设的要小得多的发现，