On the Inside

Vascular plants are the only organisms that transport water and nutrients using negative pressure. Current thinking based on the cohesion-tension theory of water transport suggests that this negative pressure is generated by the surface tension of capillarymenisci in the nanopores of fibrous cell walls, mostly in leaves, fromwhich water evaporates into intercellular spaces and then moves as water vapor through stomata into the atmosphere. The cell walls are hydraulically connected to the rest of the plant’s hydraulic system, which mainly consists of xylem tissue. The tension created in evaporating walls drives sap flow in the xylem from roots to leaves due to strong cohesion between water molecules that underlie the water potential gradient. While it is clear from physical principles and experimental microfluidic systems that evaporation from nanopores can create large negative pressures, it is not clear how plants can use this negative pressure to transport water through their xylem without constantly creating bubbles in the system, especially considering that xylem is highly complex, with many different surfaces and containing large amounts of gas. Attempts to replicate this feat in artificial systems almost invariably result in bubble formation, except under highly controlled conditions with pure water and only hydrophilic surfaces present. So how do plants transport water under negative pressure? Schenk et al. (pp. 1177–1196) show that angiosperm xylem has abundant hydrophobic surfaces similar to pulmonary surfactants. By means of transmission electron microscopy, the authors observed nanoparticles of lipid surfactants both in the pores of intervessel pit membranes and deposited on vessel wall surfaces. The nanoparticles observed in xylem sap included surfactant-coated nanobubbleswhen examined by freezefracture electron microscopy. Xylem surfactants showed strong surface activity that reduces surface tension to low values when concentrated as they are in pit membrane pores. The authors propose that xylem surfactants support water transport under negative pressure by coating hydrophobic surfaces and nanobubbles, thereby keeping the latter below the critical size at which bubbles would expand to form embolisms.