Effect of pre-existing embolism on stem vulnerability
By tracking embolism formation through a cycle of dehydration followed
by rehydration, and subsequent dehydration to complete desiccation we
show that the presence of pre-existing embolism in the xylem can lower
an apparent optical P 50. Given that the presence
of embolism in conduits is the most likely source of air seeding for
future embolism events should Ψ decline (Knipfer et al. 2015; Choat et
al. 2015b; Torres-Ruiz et al. 2016), we suspected that the presence of
pre-existing embolism would make xylem more vulnerable. Across our
diverse sample of species, the more embolism initially present in the
xylem, up to 60% embolism by area, the more negative the apparentP50 of the remaining conduits became (Figure 4).
Nevertheless, a temporal effect on embolism spreading can also be
interpreted to contribute to the more negativeP 50r values, as the slope of vulnerability curves
may illustrate the relative speed of embolism propagation, rather than
absolute embolism resistance per se. Hence, it would be interesting to
compare normal vulnerability curves with curves subject to multiple
cycles of rehydration and dehydration, which is likely to occur to
plants in the field.
The impact of pre-existing embolism on the reliability of vulnerability
curves is a widely discussed topic (Martin-StPaul et al. 2014). So much
so that flushing stems to remove embolism prior to measurements of
hydraulic conductivity on centrifuge rotors is common practice. One
explanation given for this practice is to counter the chance that
pre-existing embolism spreads rapidly through the conduits resulting in
artefacts of highly vulnerable P 50 values in a
stem (Cochard et al. 2013; Hacke et al. 2015). Our results suggest that
the presence of pre-existing embolism will occur in the most vulnerable
conduits first, thus P 50 values, determined
optically on xylem in which some conduits are already embolized, may
appear more resistant to embolism formation than if measurements were
conducted on xylem in which all of the conduits were water-filled. This
mirrors the results of a report of a comparison of vulnerability curves
measured using a centrifuge in short stem segments of long-vessel
species that had and had not been flushed (Hacke et al. 2015). While we
found that pre-existing embolism altered apparentP 50 in some species, the impact on apparentP 50 was generally a reduction of no more than
20%. In this study we never measured an R -shaped curve using the
optical method, despite initiating all experiments at high water
potentials.
Besides accurate water potential measurements at the individual conduit
level, a key unknown in our study is whether the differences in apparent
embolism resistance driven by pre-existing embolism, translate to
similarities in hydraulic vulnerability or drought tolerance. Given the
strong correlations between mean embolism resistance of xylem between
optical, microCT, pneumatic and hydraulic methods, when each is
performed carefully (Sergent et al. 2020; Gauthey et al. 2020; Pereira
et al. 2021), we hypothesize that there would be strong congruence with
hydraulic vulnerability. However, given the considerable declines in
hydraulic conductivity that would occur when a substantial percentage of
xylem is embolized (Brodribb and Cochard 2009), the effect of this
reduction, without reduced canopy transpiration, would accelerate
declines in Ψ during drought, and may not enhance survival.