Gas flow in plant microfluidic networks controlled by capillary valves

Area of Science:

• Plant Biology
• Microfluidics
• Biophysics

Background:

• Xylem vessels in trees function as natural microfluidic systems for water transport.
• Understanding air flow dynamics within xylem is crucial for plant physiology and drought response.

Purpose of the Study:

• To investigate the mechanism of air flow in Populus xylem vessels.
• To characterize the physical properties of xylem microstructure, including vessel walls and pits.
• To determine the critical pressure required for air to flow through the xylem.

Main Methods:

• Microstructural characterization using optical microscopy, transmission electron microscopy (TEM), and atomic force microscopy (AFM).
• Measurement of Young's moduli of vessel walls and pit membranes via nanoindentation and nanoflexion experiments.
• Air injection experiments to identify the critical pressure (ΔPc) for air flow.

Main Results:

• Populus xylem vessels are approximately 15 cm long with a 20 μm diameter.
• Flow between vessels occurs via ~102 pits with thin, porous membranes (310 nm).
• Pit membranes exhibit a Young's modulus of ~0.4 MPa, regardless of hydration state.
• Air flow initiates when differential pressure exceeds a critical value of 1.8 MPa (ΔPc).

Conclusions:

• The pit membrane acts as a strained porous medium, with water menisci functioning as capillary valves.
• The critical pressure (ΔPc) signifies the opening of these capillary valves, allowing air passage.
• This study provides fundamental insights into the physics of bordered pits and their role in air flow within tree xylem.