Embolism spreading in dehydrating angiosperm xylem is driven by gas
movement between embolised and sap-filled conduits. Here, we examine how
proximity to pre-existing embolism and hydraulic segmentation affect
embolism propagation. Based on the optical method, we compared xylem
embolism resistance between detached leaves and leaves attached to
branches, and between intact leaves and leaves with cut minor veins for
six species. Moreover, we directly compared the optical and pneumatic
method on detached leaves. Embolism resistance of detached leaves was
significantly lower than leaves attached to stems, except for two
species with all vessels ending in their petioles. Cutting of minor
veins showed limited embolism spreading in minor veins near the cuts
prior to major veins. Moreover, there was strong agreement in embolism
resistance between the optical and pneumatic method, with minor
differences occurring during early stages of embolism formation. We
conclude that embolism resistance may represent a relative trait,
depending on the proximity and connectivity to pre-existing embolism as
a gas source. Since embolism formation may not rely on a certain
pressure difference threshold between functional and embolised conduits,
we suggest that embolism is facilitated by pressure-driven gas
diffusion, while hydraulic segmentation can prevent embolism propagation
by reducing gas diffusion.