Relax and refill: xylem rehydration prior to hydraulic measurements favours embolism repair in stems and generates artificially low PLC values

Abstract

Diurnal changes in percentage loss of hydraulic conductivity (PLC), with recorded values being higher at midday than on the following morning, have been interpreted as evidence for the occurrence of cycles of xylem conduits' embolism and repair. Recent reports have suggested that diurnal PLC changes might arise as a consequence of an experimental artefact, that is, air entry into xylem conduits upon cutting stems, even if under water, while under substantial tension generated by transpiration. Rehydration procedures prior to hydraulic measurements have been recommended to avoid this artefact. In the present study, we show that xylem rehydration prior to hydraulic measurements might favour xylem refilling and embolism repair, thus leading to PLC values erroneously lower than those actually experienced by transpiring plants. When xylem tension relaxation procedures were performed on stems where refilling mechanisms had been previously inhibited by mechanical (girdling) or chemical (orthovanadate) treatment, PLC values measured in stems cut under native tension were the same as those measured after sample rehydration/relaxation. Our data call for renewed attention to the procedures of sample collection in the field and transport to the laboratory, and suggest that girdling might be a recommendable treatment prior to sample collection for PLC measurements.

Direct X-Ray Microtomography Observation Confirms the Induction of Embolism upon Xylem Cutting under Tension

“…However, recent publications have provided evidence that some measurement techniques used to evaluate the hydraulic function and vulnerability to cavitation of plant organs may be prone to artifacts (Sperry et al, 2012;Torres-Ruiz et al, 2014;Trifilò et al, 2014). The discovery of these artifacts has raised questions regarding the reliability of some previously published plant hydraulics data, in particular data relating to the refilling of embolized xylem conduits while the xylem is under tension.…”

“…Whereas Scoffoni and Sack (2014) showed that the artifact described by Wheeler et al (2013) has no impact on leaf xylem hydraulic conductance, there is some uncertainty about its importance in stems or shoots (Trifilò et al, 2014;Venturas et al, 2014). The results of Wheeler et al (2013) indicate that more embolism could be induced by cutting samples that are under midrange xylem tension (e.g.…”

Xylem embolism refilling and resilience against drought‐induced mortality in woody plants: processes and trade‐offs

“…(2) To avoid artefactual embolism induction caused by the cutting of a branch, while the xylem is under tension, rehydration/relaxation of samples has been suggested, followed by re-cutting under degassed water (Wheeler et al 2013), although this artefact has not been found by others (Venturas et al 2015). Alternatively, the relaxation procedure has been shown to introduce other potential errors related to rehydration-induced embolism refilling (Trifilo`et al 2014b). Recommendations for standardizing measurements of vulnerability curves have recently been described (Torres-Ruiz et al 2014).…”

“…Future research should carefully evaluate the occurrence and magnitude of cutting or relaxation artefact in species investigated for their refilling capability. Trifilo`et al (2014b) suggested the use of refilling inhibitors (e.g. orthovanadate) to investigate the impact of cutting and relaxation on PLC values.…”

Annual patterns of xylem embolism in high-yield apple cultivars

“…In the laboratory, branches were repeatedly cut from both sites alternatively under water. In an earlier study (Beikircher and Mayr 2015), this harvesting and sampling protocol proved to be appropriate for preserving the hydraulic state and avoid artefactual embolisation as well as refilling (Trifilò et al 2014). Before June, shoots developed in the previous season were used for measurements, whereas from June onward, we used the respective current-year shoots.…”