Turgor loss in vessel-associated parenchyma cells increases xylem vulnerability to embolism

Drought-induced depletion of non-structural carbohydrates has been reported to affect xylem hydraulic vulnerability, which in turn is frequently correlated with water potential at turgor-loss point. Given that non-structural carbohydrate depletion can impair osmoregulation, we hypothesised that vessel-associated parenchyma cells (VACs) that undergo drought-induced turgor loss and plasmolysis could facilitate gas movement and the formation of xylem embolism. Plasmolysis was induced in wood parenchyma of Populus nigra stems of mature trees and potted plants by radial injection or axial perfusion with a polyethylene glycol solution at low osmotic potential. The effect of polyethylene glycol on embolism resistance was assessed using the gas injection technique followed by classic hydraulic quantification of embolism, as well as with flow-centrifuge measurements. Light and transmission electron microscopy confirmed the occurrence of plasmolysis of VACs in osmotically treated samples, while hydraulic measurements revealed an increase in xylem vulnerability to embolism upon induction of plasmolysis, raising the loss of hydraulic conductivity by ∼20-40%. The results therefore support the hypothesis that the maintenance of cell turgor in VACs is critical for xylem hydraulic integrity under drought. We speculate that plasmolysis of VACs could promote gas movement to functional vessels via vessel-parenchyma pits, increasing the likelihood of embolism propagation.