Xylem vulnerability to embolism represents an essential trait for the evaluation of the impact of hydraulics in plant function and ecology. The standard centrifuge technique is widely used for the construction of vulnerability curves, although its accuracy when applied to species with long vessels remains under debate. We developed a simple diagnostic test to determine whether the open-vessel artefact influences centrifuge estimates of embolism resistance. Xylem samples from three species with differing vessel lengths were exposed to less negative xylem pressures via centrifugation than the minimum pressure the sample had previously experienced. Additional calibration was obtained from non-invasive measurement of embolism on intact olive plants by X-ray microtomography. Results showed artefactual decreases in hydraulic conductance (k) for samples with open vessels when exposed to a less negative xylem pressure than the minimum pressure they had previously experienced. X-Ray microtomography indicated that most of the embolism formation in olive occurs at xylem pressures below -4.0 MPa, reaching 50% loss of hydraulic conductivity at -5.3 MPa. The artefactual reductions in k induced by centrifugation underestimate embolism resistance data of species with long vessels. A simple test is suggested to avoid this open vessel artefact and to ensure the reliability of this technique in future studies.

BIOGECO INRA University of Bordeaux 33615 Pessac France

CNRS University of Bordeaux UMS 3626 Placamat F 33608 Pessac France

Faculty of Forestry and Wood Sciences Czech University of Life Sciences Kamýcká 129 165 00 Praha 6 Suchdol Czech Republic

INRA UR629 Ecologie des Forêts Méditerranéennes F 84914 Avignon France

Instituto de Recursos Naturales y Agrobiología de Sevilla Avenida Reina Mercedes 10 41012 Sevilla Spain

PIAF INRA University of Clermont Auvergne 63100 Clermont Ferrand France

Synchrotron SOLEIL L'Orme de Merisiers 91190 Saint Aubin BP48 Gif sur Yvette Cedex France

Ulm University Institute of Systematic Botany and Ecology Albert Einstein Allee 11 89081 Ulm Germany

Western Sydney University Hawkesbury Institute for the Environment Richmond NSW 2753 Australia

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