Embolism resistance of conifer roots can be accurately measured with the flow-centrifuge method

Abstract

Resistance to embolism of conifer branches has commonly been studied with the flow-centrifuge technique (Cavitron) to carry out routine measurements. The aim of this study was to test the accuracy of the Cavitron for measurements on conifer roots. Based on earlier findings, it was suggested that the application of the flow-centrifuge technique to roots may not be free from artefacts due to potential torus aspiration when the pressure gradient between the sample ends is high. Here, three different protocols were used to obtain vulnerability curves in Pinus pinaster and Pseudotsuga menziesii roots: no water pressure gradient, and low and high water pressure gradient. In addition water extraction curves were obtained to only estimate the water released from conduits by embolism. Water extraction curves showed no water release caused by embolism before -1.3 and -2.5 MPa for P. pinaster and P. menziesii, respectively. The results illustrated discrepancies between these protocols: roots measured with the high and low pressure gradient protocols appeared erroneously more vulnerable to embolism than roots measured with no pressure gradient. In addition, pit anatomical observations of roots showed non-punctured tori and a high flexibility of the margo, which may increase the risk of torus aspiration. All together these results suggest that the early loss of hydraulic conductivity observed for the low and high pressure gradient protocols was not due to embolism but rather to a torus aspiration artefact when the pressure gradient is too high. We conclude that the no pressure gradient protocol provides a suitable method and that high vulnerability to embolism reported in previous studies for conifer roots should be interpreted with caution in the light of our findings.