Challenges in understanding air-seeding in angiosperm xylem

High porosity with tiny pore constrictions and unbending pathways characterize the 3D structure of intervessel pit membranes in angiosperm xylem

“…According to the Young Laplace equation, the pressure difference forcing a bubble through a 20 nm pore, assuming a contact angle of zero (Caupin, Cole, Balibar, & Treiner, ; Meyra, Kuz, & Zarragoicoechea, ), and a pore shape correction factor of 0.5 (Schenk et al, ), would be 7.2 MPa in pure water. Because surface‐active substances, such as phospholipids, are known to occur in xylem sap and to be associated with pits (Jansen et al, ; Schenk et al, ; Schenk et al, ), the surface tension inside pores is likely to be much reduced. If the surface tension of an air‐water meniscus is reduced to 24 mJ m −2 , which is a typical equilibrium surface tension for phospholipid monolayers (Lee, Kim, & Needham, ), a meniscus could pass through a 20 nm pore with a shape correction factor of 0.5 under a pressure difference of 2.4 MPa.…”

“…Sample preparation for TEM was performed as described above, but without applying OsO 4 treatment. Since no OsO 4 was used as post‐fixative, pit membranes were highly transparent (Jansen et al, ; Schenk et al, ; Schenk, Espino, Rich‐Cavazos, & Jansen, ), and individual gold particles of all sizes could easily be observed as circular, electron dense structures. OsO 4 treatment, however, results in binding of Os to unsaturated fatty acid chains of lipids (Riemersma, ), which results in dark, electron dense particles associated with pit membranes.…”

“…the pore volume fraction in a pit membrane) represents an important characteristic for hydraulic resistance and vulnerability to air‐seeding. The geodesic tortuosity (τ, Figure b) can be defined as the ratio of the mean shortest flow path length to the thickness of the porous medium (Neumann, Stenzel, Willot, Holzer, & Schmidt, ) and quantifies the geometric complexity of pores in pit membranes (Jansen et al, ). If the mean shortest flow path length is equal to the thickness of the porous medium, then the tortuosity value is 1.…”

“…The constrictivity (β) is calculated as: Hence, a lower constrictivity value indicates that more constrictions occur in the pore space. Determining the constrictivity of pit membranes is important for gas bubble snap‐off inside pores (Jansen et al, ; Kovscek & Radke, ; Roof, ).…”

“…This study aims to investigate the porous medium characteristics of intervessel pit membranes by applying a modelling approach that is based on comparing the thickness of fresh with completely dried and shrunken pit membranes. Earlier work on pit membranes (Jansen et al, ; Jansen et al, ; Li et al, ; Schmid & Machado, ; Zhang et al, ) allowed us to develop a shrinkage model to estimate the mean pore space between microfibril aggregates in fresh, fully hydrated pit membranes and dried, shrunken pit membranes. It is expected that porosity values of pit membranes are relatively high, similar to other fibrillar, non‐woven porous media in nature (Shou, Fan, & Ding, ).…”