Xylem resistance to embolism: presenting a simple diagnostic test for the open vessel artefact

Abstract

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.

Mapping xylem failure in disparate organs of whole plants reveals extreme resistance in olive roots

“…Cross‐validation of this technique with hydraulic methods in stems (Brodribb et al ., ) and leaves (Brodribb et al ., ) provides confidence that the proxy measurement of ‘area cavitated’ faithfully represents the percentage loss of hydraulic conductance measured by flow methods. In particular, our average P 50 for stems (−5.67 MPa) agrees with the average P 50 obtained by hydraulic methods (benchtop dehydration method) and X‐ray micro‐CT in olive species (−5.70 MPa) (Ennajeh et al ., ; Torres‐Ruiz et al ., , ), validating the OV method, at least at stem level, in olive, a narrow, long‐vesseled species. The optical P 50 value did not agree with the much higher vulnerability for olive stems produced by methods involving stem excision, supporting the probability of artefacts associated with these ex situ methods for measuring species with relatively long vessels such as olive (Torres‐Ruiz et al ., , ).…”

Declining root water transport drives stomatal closure in olive under moderate water stress

“…Our results identify dynamic root hydraulic conductance during water stress as an important driver of early stomatal closure in olive plants. Our novel hydraulic method allowed us to observe sharp declines in whole root K , including the soil–root interface, occurring over the same range of water potentials as stomatal closure, far above those required to trigger xylem cavitation (Torres‐Ruiz et al ., ; Rodriguez‐Dominguez et al ., ). Our findings indicate that the most important changes in the hydraulic system of these olive plants during moderate water stress did not occur in leaves or stems, and did not involve cavitation.…”

Climatic limits of temperate rainforest tree species are explained by xylem embolism resistance among angiosperms but not among conifers

“…For angiosperm species, some samples per species were used to test the presence of open vessels (Torres‐Ruiz et al , ) by injecting air into stems at 2 bar at one end. Samples from three species ( Metrosideros umbellata , Melicytus ramiflorus and Myrsine australis ) had open vessels in 27 cm long samples, provided r‐shaped curves and were therefore discarded.…”