Nanobubbles: a new paradigm for air-seeding in xylem

•Nanobubbles develop as air–water menisci move through pores between xylem conduits. •Radius-dependent surface tension of surfactants stabilizes nanobubbles under tension. •Surfactants may decrease, rather than increase, xylem embolism vulnerability. •Nanobubbles provide a new paradigm for hydraulic failure of plant water transport. Long-distance water transport in plants relies on a system that typically operates under negative pressure and is prone to hydraulic failure due to gas bubble formation. One primary mechanism of bubble formation takes place at nanoporous pit membranes between neighboring conduits. We argue that this process is likely to snap off nanobubbles because the local increase in liquid pressure caused by entry of air-water menisci into the complex pit membrane pores would energetically favor nanobubble formation over instant cavitation. Nanobubbles would be stabilized by surfactants and by gas supersaturation of the sap, may dissolve, fragment into smaller bubbles, or create embolisms. The hypothesis that safe and stable nanobubbles occur in plants adds a new component supporting the cohesion-tension theory. Long-distance water transport in plants relies on a system that typically operates under negative pressure and is prone to hydraulic failure due to gas bubble formation. One primary mechanism of bubble formation takes place at nanoporous pit membranes between neighboring conduits. We argue that this process is likely to snap off nanobubbles because the local increase in liquid pressure caused by entry of air-water menisci into the complex pit membrane pores would energetically favor nanobubble formation over instant cavitation. Nanobubbles would be stabilized by surfactants and by gas supersaturation of the sap, may dissolve, fragment into smaller bubbles, or create embolisms. The hypothesis that safe and stable nanobubbles occur in plants adds a new component supporting the cohesion-tension theory.