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.