Xylem sap residue in cut-open conduits can affect gas discharge in pneumatic experiments

Summary Considerable progress has been made in understanding the mechanisms of embolism formation based on the pneumatic method, which relies on gas discharge measurements. Here, we test the assumption that cut-open conduits are gas-filled when samples are cut at high water potentials. We performed vulnerability curves (VC) with the Pneumatron and analysed sap extraction from cut-open vessels in Citrus branches, while the optical method was applied as a reference method. VCs of 11 additional angiosperms were analysed to generalise our findings. We found an increase in gas discharge during early stages of dehydration, which affected the VC of Citrus . Xylem sap was not absorbed immediately by surrounding tissue in cut Citrus branches. The gas amount discharged increased until all sap residue was absorbed, which was near the turgor loss point. By analysing the slope of VCs, we could correct pneumatic VC, as evidenced by the strong agreement in embolism resistance between the pneumatic and the optical method. Since residual sap in cut-open conduits of some species could slightly reduce embolism resistance in some species, we propose to apply an easy correction for this novel artefact. Automated measurements with a Pneumatron are also required because of its high time resolution.