Discussion
The dehydration-rehydration-dehydration experiments conducted here
indicate that embolism spreading is not random. While the water
potentials of individual conduits remain impossible to determine, our
experimental data suggests that pit membranes between adjacent conduits
appear to work as fairly efficient safety valves when dehydration levels
are rather low, ensuring that individual conduits have a high fidelity
to a specific water potential threshold at which gas will invade the
conduit. However, once sufficient embolism has accumulated in the xylem
the spreading of embolism into water filled conduits does not appear to
be strictly driven by a fixed pressure threshold between neighbouring
conduits. When more than 60% of the xylem is embolized, gas spreads
rapidly into water-filled conduits upon dehydration. This could be due
to the amount of embolised conduits present, but also gas movement
across embolised or sap-filled conduit lumina, pit membranes, and cell
walls triggering embolism. These results have important implications for
understanding the nature of embolism formation during dehydration as
well as the impact of pre-existing embolism on the determination of
relative embolism resistance.