Dynamic Surface Tensions of Nanobubbles in Plant Xylem: When are they Stable?

Xylem sap exists in a state some have described as “doubly” metastable[1]: liquid water is transported from root to leaf under negative pressure, and, in some climates, below its freezing point. Sub-100 nm nanobubbles may be injected into the xylem liquid through pit membranes[2], becoming coated with Phospho- and Glycolipids in the process. Their surface properties, and therefore fate within the tree hydraulic system, remain largely unexplored.

In this work, we have used molecular dynamics simulations to produce surface tension – area isotherms of biologically relevant lipid monolayers, as a function of both temperature and negative pressure (i.e. dynamic surface tensions).

We find that glycolipid monolayers resist expansion proportionally to the rate of expansion[3]. Their surface tension increases with the tension applied, stabilising the bubble with respect to embolism. In contrast, a typical phospholipid rapidly condenses into more dense lamellar-like phase, rendering it highly resistant to tensions as high as -3.5 MPa. Mixed monolayers of the two exhibit hybrid behavior, as the glycolipids' larger head group disrupts the more ordered phase of the phospholipid. Finally, it is observed that increasing temperature also increases surface tension, at a given surface area.

[1] Lintunen, A., Hölttä, T., & Kulmala, M. (2013). Anatomical regulation of ice nucleation and cavitation helps trees to survive freezing and drought stress. Scientific Reports, 3, 1–7. https://doi.org/10.1038/srep02031

[2] Schenk, H. J., Steppe, K., & Jansen, S. (2015). Nanobubbles: a new paradigm for air-seeding in xylem. Trends in Plant Science, 20(4), 199–205. https://doi.org/10.1016/j.tplants.2015.01.008

[3] Ingram, S., Salmon, Y., Lintunen, A., Hölttä, T., Vesala, T., & Vehkamäki, H. (2021). Dynamic Surface Tension Enhances the Stability of Nanobubbles in Xylem Sap. Frontiers in Plant Science, 12. https://doi.org/10.3389/fpls.2021.732701