Geometrical and physicochemical considerations of the pit membrane in relation to air seeding: the pit membrane as a capillary valve

Abstract

A theoretical treatment of some of the factors influencing air seeding at the pit membranes of xylem vessels is given. Pit membrane structure, viewed as a three-dimensional mesh of intercrossing fibrils, and vulnerability to water-stress-induced air seeding are examined in the context of the Young-Laplace equation. Simple geometrical considerations of the porous membrane show that the vapor-liquid interface curvature radius is a function of fiber-fiber distance, fiber radius, wetting angle and position of the wetting line. Air seeding (maximum pressure) occurs at the minimum curvature radius, therefore air seeding is not simply determined by the fiber-fiber distance but is a function of the geometry of the pit membrane and of physicochemical quantities like surface tension and wetting angle. As a consequence of considering a wetting angle different from zero, the minimum curvature radius becomes larger than half the fiber-fiber distance. The present model considers that, for a given pressure difference at the pit membrane, all local interface curvatures are the same. In this sense, pit membranes work as variable capillary valves that allow or prevent air seeding by adjusting local curvatures and interface positions relative to the pore-forming fibers, following the pressure differences across the membranes. The theoretical prediction for the air seeding threshold is consistent with recent experimental data for angiosperm trees.

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

“…How likely is air‐seeding through a 20 nm pore constriction, which is close to the constriction size estimations based on our shrinkage model and gold perfusion experiments? 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.…”

Bordered pits in xylem of vesselless angiosperms and their possible misinterpretation as perforation plates

“…Additional attention is given to the potential occurrence of lipids in intertracheid pit membranes as reported in vesselbearing angiosperms, which would have important consequences for air-seeding (Schneider et al 1999;Meyra et al 2007;Westhoff et al 2008;Schenk et al 2017). We will also examine the presence of an amorphous layer, potential differences in intertracheid pit membrane structure between earlywood and latewood and the occurrence of tracheid dimorphism, which was reported in Tetracentron sinense (Suzuki et al 1991).…”

Xylem Surfactants Introduce a New Element to the Cohesion-Tension Theory

“…6), it is not possible to predict how low it may be in a given pore. To give an example for a possible effect of lipid surfactants on air seeding, a surface tension of 24 mJ m 22 (approximately the equilibrium tension for phospholipids; Lee et al, 2001), an effective contact angle of zero, and a pore shape correction factor k of 0.5 for a fibrous medium (Meyra et al, 2007;Caupin et al, 2008;Schenk et al, 2015) would require only a pressure difference of 0.8 MPa to force a meniscus through a pore of 30-nm radius or of 0.4 MPa through a pore of 60-nm radius. These are probably realistic radii for the largest pores in hydrated pit membranes in vivo (Jansen et al, 2009;Lens et al, 2011) and normal xylem pressures under unstressed conditions.…”