Embolism spreading may not be only pressure-driven
The more than 1 MPa difference in PEP50 values between detached leaves and leaves connected to stems provided strong evidence that embolism formation may not always be pressure driven, as traditionally assumed based on the air-seeding hypothesis and the Young-Laplace equation (Sperry and Tyree, 1988; Choat et al. , 2008). The finding that embolism formation in xylem tissue from the same organ of a species may occur under different xylem water potentials is not entirely new, and in line with earlier differences in embolism resistance between intact plants and cut plants (Choat et al. , 2010; Torres-Ruiz et al. , 2015; Lamarque et al. , 2018). For instance, a ca. 4 MPa difference in P50 was found for Laurus nobilis based on microCT observation of cut branches and intact seedlings (Lamarque et al. , 2018; Nardini et al. , 2017). Similar to our findings, Skelton et al. (2018) compared the vulnerability curves of cut branches and intact plants based on the optical method for Quercus wislizenii , and found a -1.5 MPa difference in P50 between leaves attached to a long, cut branch, and leaves from an intact plant. The finding that cut plant material can be more vulnerable to embolism spreading than intact plants raises concerns about embolism resistance measurements of plant material samples with pre-existing embolism, the possible induction of embolism due to cutting, and the application of the bench dehydration method on cut plant material (Sperry and Tyree, 1988).
An important question concerns the exact triggering mechanism behind embolism, which cannot be fully addressed based on the available evidence. However, since embolism spreading may not be pressure-driven, we speculate that gas diffusion across pit membranes will take place well before embolism formation has started. Although mass flow is theoretically 105 times faster than diffusion, gas diffusion in xylem is much faster and more common than mass flow. The main reason seems to be that gas diffusion takes place continuously over very large areas, while mass flow according to the air-seeding hypothesis relies on gas movement through multi-layered, tiny pore constrictions of mesoporous pit membranes. Further research is clearly needed to investigate whether or not gas diffusion may contribute to (super)saturation of xylem sap (Schenk et al. , 2016), how gas-water interfaces are affected by the dynamic surface tension of xylem sap lipids (Yang et al. , 2020), and how surfactant-coated nanobubbles may affect the gas concentration of xylem sap (Schenket al. , 2015, 2017; Jansen et al. , 2018; Park et al. , 2019).