Xylem Dysfunction: When Cohesion Breaks Down

Abstract

Xylem is an unusual tissue. The conduits (vessels or tracheids) are alive and filled with water and cellular organelles from the time they divide through growth and differentiation. When they are mature they die to become functional. Conduits can remain functional for just a few days or for more than 100 years, but the first step towards a state of permanent dysfunction probably involves a state of embolism. Freezing can induce embolism because air comes out of solution as water turns to ice. Drought stress can induce embolism because xylem pressure, P _x , becomes negative enough during drought to induce a cavitation. Simple mechanical damage can also induce this first stage of xylem dysfunction. However, embolism dysfunction is not always permanent. Sometimes embolisms can be repaired daily and sometimes by the next growth season. In addition, embolized conduits are not totally dysfunctional, since even embolized conduits retain some water and the amount of water retention increases and decreases with P _x ,. So embolized conduits still function as water-storage organs. In this chapter we will review the mechanisms of xylem dysfunction, vulnerability curves, and mechanisms of embolism repair (removal).