Reversible Leaf Xylem Collapse: A Potential "Circuit Breaker" against Cavitation

Area of Science:

• Plant physiology
• Xylem function
• Leaf hydraulics

Background:

• Angiosperms face xylem dysfunction risks, especially cavitation, impacting water transport.
• Leaf vein structure plays a critical role in water distribution and stress tolerance.

Purpose of the Study:

• To investigate a novel form of xylem dysfunction in red oak leaves.
• To understand the role of minor vein collapse in leaf water relations and cavitation avoidance.

Main Methods:

• Cryo-scanning electron microscopy (Cryo-SEM) to visualize xylem conduit structure.
• Dehydration and rehydration experiments to assess vein collapse and recovery.
• Modeling of transpiration transients to evaluate the functional impact of vein collapse.

Main Results:

• Reversible collapse of xylem conduits in minor leaf veins was observed between -2 and -3 MPa.
• Cavitation was negligible in these collapsing veins, indicating a protective mechanism.
• Cell wall collapse depended on mechanical constraints from neighboring cells, not just individual properties.
• Minor vein collapse and mesophyll capacitance buffered major veins against cavitation during transpiration.

Conclusions:

• Minor vein collapse is a novel, reversible xylem dysfunction in angiosperms.
• This mechanism contributes to preventing cavitation, enabling transpiration near critical water potentials.
• Vein collapse is a significant factor in leaf hydraulic strategy, particularly for plants with large stomatal apertures.