When plants experience drought, gas bubbles (embolisms) can form that block the vascular tubes (xylem) responsible for carrying water from roots to leaves. These blockages cause a plant to weaken and eventually die.
Grapevines are very efficient at repairing these vascular blockages, but the underlying mechanism remains unclear. In the springtime, after being freshly pruned, grapevines drip because their root systems generate positive pressures that push water and minerals upward. This root pressure has been assumed to play a role in embolism repair by forcing gas out of empty xylem tubes.
To determine whether embolism removal is possible without root pressure, researchers used ALS Beamline 8.3.2 to obtain 3D microtomographic images of short grapevine stem segments detached from roots and leaves. They also investigated the metabolic activity of the soft tissue (parenchyma) around the xylem using fluorescence light microscopy.
The results provided direct, visual evidence that embolism repair in excised stem samples is qualitatively identical to the process observed in intact plants, driven by water droplets emerging from the living cells wrapped around the dead xylem tubes.
Overall, the results emphasize that root pressure is not required as a driving force for vessel refilling, and care should be taken when performing hydraulics measurements on excised plant organs containing living vessel-associated tissue, because the vessel behavior may not be static.
Three-dimensional microtomographic volume rendering showing the structure of dead xylem vessels with connections to vessel-associated tissue in a grapevine stem.
Work performed at ALS Beamline 8.3.2.
T. Knipfer, I.F. Cuneo, C.R. Brodersen, and A.J. McElrone, “In Situ Visualization of the Dynamics in Xylem Embolism Formation and Removal in the Absence of Root Pressure: A Study on Excised Grapevine Stems,” Plant Physiol. 171, 1024 (2016).