The maintenance of cell turgor in wood parenchyma is critical for xylem hydraulic safety: experimental evidence in Populus nigra (L.)

Non-structural carbohydrates (NSCs) are involved in several plant functions such as growth, metabolism, transport, osmoregulation and defence, but recent evidence supports their putative role in maintaining plant hydraulic integrity under drought. Indeed, stem NSC-depleted Populus nigra (L.) saplings showed an increased vulnerability to xylem embolism formation, but a mechanistic explanation for this phenomenon is missing. We hypothesised that low NSC content in wood parenchyma might impede osmoregulation under drought, leading to plasmolysis. Consequently, an earlier turgor loss could lead to formation of air gaps between xylem vessels and adjacent plasmolysed parenchyma cells that would serve as additional gas-entry points for xylem embolism build-up. To test this hypothesis, plasmolysis was induced in xylem parenchyma cells by applying polyethylene glycol (PEG) solutions at known osmotic potentials on branch segments of adult trees, and inducing embolism with a pressure collar. The results showed that PEG-treated branches had an increased stem xylem vulnerability compared to controls. These results highlight two key questions deserving further investigation: (1) what is the role of xylem vessel-associated cells and of conduit-parenchyma cell pit characteristics in embolism propagation in the xylem? (2) What is the role of NSC reserves in shaping plant hydraulic behaviour and plant survival under drought?