The Fluid–Solid Interaction in the Nonlinear Pressure–Flux Relationship of Bordered Pits in Oriental Arborvitae (Platycladus orientalis)

The nonlinear pressure–flux relationship in the xylem of a conifer is attributed to the fluid–solid interaction within the bordered pits. However, the fluid–solid interactions between the torus–margo structure and the water flow within the pit lack comprehensive understanding. Herein, a fluid–solid interaction model was developed including the deformation of the torus–margo and the flow of water transportation. Nine pit samples were reconstructed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) pictures. Fluid–solid coupling models for pits in the stems of oriental arborvitae (Platycladus orientalis) were developed. The deflection of the torus was roughly proportional to the pressure difference between adjacent tracheids, while the pit resistance exhibited a considerable nonlinear increase. From 250 to 1750 Pa, the pit resistance increased from 4.1466 × 1016 Pa·s/m3 to 8.8344 × 1016 Pa·s/m3. The pit resistance decreased, and the pit’s ability to regulate water flow enhanced when the pit diameter increased. Both the pit resistance and the pit’s ability to regulate water decreased when the pit depth increased. The decrease in Young’s modulus for the margo promoted the nonlinear pressure–flux relationship in bordered pits. The findings provide theoretical evidence for the nonlinear relationship between pressure and flux in bordered pits, as well as for the prevention of gas bubble transit through a bordered pit during tracheid cavitation. The passive hydraulic regulation of bordered pits could increase flow resistance and reduce the water flow rate in the xylem, inhibiting tree transpiration.