The optical method and Pneumatron show close similarity in
measuring embolism resistance
We found a strong agreement between the optical method and the
Pneumatron, with no significant difference between either
P12, P50 or P88 values
obtained from the optical method and the Pneumatron in five deciduous
species (Figure 4, Table S2). This finding is in line with the results
from Pereira et al. (2020a) for Eucalyptus camaldulensis ,
and supports the hypothesis that the pneumatic method measures gas
extraction from embolism events in intact vessels (Jansen et al. ,
In press; Yang et al. , submitted). The fast and straightforward
approach of taking pneumatic methods on small samples such as individual
leaves makes the Pneumatron device also suitable for field observations
and embolism resistance measurements at the intraspecific and intratree
level.
While a high coefficient of determination (R2=0.91) indicated good agreement between PEP50 and
PAD50 values (Fig. 5b), the vulnerability curves with
the pneumatic method were less steep than with the optical method for
four species studied, except for L. tulipifera . A somewhat
weaker, but still significant correlation was found between the
PEP12 and PAD12 values based on both
methods (Fig. 5a). It is possible that the presence of gas in cut-open
vessels affects the process of embolism spreading as discussed above,
and this may explain why the percentage of gas discharged by the
Pneumatron was slightly higher than the percentage of the cumulative
embolised pixels during early stages of dehydration in four out of five
species studied (Figure 3, Figure 5A). A similar finding was reported in
Pereira et al. (2020a). However, the opposite was found forP. avium , which may have slower gas diffusion due to its thick
pit membranes as compared to the other species (Kaack et al. ,
2020). If intact vessels become embolised, but gas diffusion across pit
membranes is slow, atmospheric pressure will not be quickly reached in a
recently embolised vessel (Wang et al. , 2015a, b).
The xylem area selected to apply the optical method was deliberately
chosen in the upper part of the leaf blade, where vessels in the leaf
veins are separated from cut vessels at the petiole end by at least one
and most likely various intervessel walls, as shown based on the maximum
vessel length in petioles. Since the Pneumatron extracts gas from the
petiole end, the tight similarity in embolism resistance between both
methods suggests that the gas extracted with the Pneumatron comes from
intact, embolised conduits. It is possible that there could be some
overlap between vessels from which gas is extracted with the Pneumatron
and those that are visualised with the optical method (Fig. 6). Yet, it
remains unclear over how many end walls the Pneumatron is able to
extract gas. We speculate that this number of end walls depends at least
partly on the pit membrane thickness, the complete or partial hydration
of the pit membrane, and whether or not porous medium characteristics of
interconduit pit membranes change during dehydration, since these would
determine gas diffusion considerably (Crombie et al. , 1985; Kaacket al. , 2019; Zhang et al. , 2020).