Conclusion
Our experiments suggest that in the absence of considerable embolism in
the xylem individual conduits have a specific Ψ threshold at which
embolism will form. However, as embolism accumulates in the xylem the
likelihood of gas spreading from embolized to neighbouring water-filled
conduits increases, leading to a rapid spreading of embolism on
dehydration. We believe this is due to pit membrane anatomy, combined
with the increased anatomical heterogeneity of angiosperm xylem, which
provides both scope for variation in embolism resistance as well as a
physical protection from gas movement from embolised conduits to
sap-filled ones when there is not considerable gas presence in the
xylem. Our results have important implications for understanding the
impact of pre-existing embolism on the accurate determination of xylem
vulnerability, with pre-existing embolism apparently increasing xylem
embolism resistance. The impact of this effect of pre-existing embolism
on drought survival through repeated drought events remains to be
tested.