The stability enigma of hydraulic vulnerability curves: addressing the link between hydraulic conductivity and drought-induced embolism

Abstract

Maintaining xylem water transport under drought is vital for plants, but xylem failure does occur when drought-induced embolisms form and progressively spread through the xylem. The hydraulic method is widely considered the gold standard to quantify drought-induced xylem embolism. The method determines hydraulic conductivity (Kh) in cut branch samples, dehydrated to specific drought levels, by pushing water through them. The technique is widely considered for its reliable Kh measurements, but there is some uncertainty in the literature over how to define stable Kh and how that relates to the degree of xylem embolism formation. Therefore, the most common setup for this method was extended to measure four parameters: (i) inlet Kh, (ii) outlet Kh, (iii) radial flow from xylem to surrounding living tissue and (iv) the pressure difference across the sample. From a strictly theoretical viewpoint, hydraulic steady state, where inflow equals outflow and radial flow is zero, will result in stable Kh. Application of the setup to Malus domestica Borkh. branches showed that achieving hydraulic steady state takes considerable time (up to 300 min) and that time to reach steady state increased with declining xylem water potentials. During each experimental run, Kh and xylem water potentials dynamically increased, which was supported by X-ray computed microtomography visualizations of embolism refilling under both high- (8 kPa) and low-pressure (2 kPa) heads. Supplying pressurized water can hence cause artificial refilling of vessels, which makes it difficult to achieve a truly stable Kh in partially embolized xylem.

Visual and hydraulic techniques produce similar estimates of cavitation resistance in woody species

“…Although the bench dehydration technique is labour‐intensive, it is regarded as the most reliable hydraulic technique for measurement of cavitation resistance and provides a direct measure of impacts on hydraulic conductance. Even so, we note that a number of sources of experimental error have been identified with the bench dehydration method, including those associated with excision of plant material (Wheeler et al ., 2013) and measurement of xylem flow rates on the bench (Espino & Schenk, 2011; De Baerdemaeker et al ., 2019). In this context, the close agreement between hydraulic methodology and in situ visual techniques suggests that all techniques tested here produce reliable estimates of cavitation resistance if the appropriate precautions are taken.…”

A user manual to measure gas diffusion kinetics in plants: Pneumatron construction, operation and data analysis

“…The datasets generated for this study can be found in the Github https://github.com/Pneumatron. (Sperry et al, 1988;Bréda et al, 1993;Wheeler et al, 2013;De Baerdemaeker et al, 2019;Bonetti et al, 2020) Centrifuge self-construction is technically highly challenging; hydraulic artefacts; destructive (Alder et al, 1997;Cochard et al, 2005;Torres-Ruiz et al, 2014Wang et al, 2014;Peng et al, 2019) Air and not straightforward (Milburn and Johnson, 1966;Tyree et al, 1984;Nolf et al, (Hochberg et al, 2016;Bouda et al, 2019)…”

Drought-Induced Xylem Embolism Limits the Recovery of Leaf Gas Exchange in Scots Pine

“…Observations of embolism could also be hampered by artifacts associated with hydraulic conductivity measurements, such as bubble formation by cutting of xylem under negative pressure, and artificial refilling of embolized conduits (Wheeler et al, 2013;De Baerdemaeker et al, 2019). For this reason, nonintrusive methods provide a welcoming alternative to visualize the hydraulic status of xylem conduits in vivo.…”