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

Diurnal changes in percentage loss of stem hydraulic conductivity (PLC), with recorded values being higher at midday than on the following morning, have been generally interpreted as evidence for the occurrence of cycles of xylem conduits' embolism and repair. Recent reports have suggested that such diurnal PLC changes might arise as a consequence of an experimental artefact i.e. 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 favours xylem refilling and embolism repair, thus leading to PLC values erroneously lower than those actually experienced by transpiring plants. In fact, when tension relaxation was 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 where the same as those measured after sample rehydration/relaxation. Our data urgently call for caution during to 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.