The optical method and Pneumatron showed strong 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. (2019) for Eucalyptus camaldulensis ,
and supports the hypothesis that the pneumatic method relies largely on
gas extraction from embolism events in intact vessels (Jansen et
al. , In press). The fast and straightforward approach of taking
pneumatic methods on small samples such as individual leaves make this
method also suitable for field observations and further work at the
intraspecific and intratree level.
While a high coefficient of determination (R2=0.91) indicated good agreement between P50 values
obtained from two methods (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 P12values based on both methods (Fig. 5a). 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 the intact vessels that are
equally affected by the presence of gas from the cut-open vessels, or
close to those that are observed with the optical method (Fig. 6). It is
possible that there could be an overlapping area with embolism measured
in intact vessels based on both methods (Fig. 6). Yet, it is 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; Zhanget al. , 2020; Kaack et al. , 2019).
It is possible that the pressure and humidity inside an embolised vessel
affects the process of embolism spreading, with a more efficient
propagation of embolism spreading from a cut vessel to an intact one
than between two intact ones. 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), and a similar
trend was found in Pereira et al. (2019). However, the opposite
was found for P. avium , which may have slower gas diffusion due
to its thick pit membranes as compared to the other species. 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). Interestingly, the
amount of gas extracted with the Pneumatron is considerably lower than
the amount of gas that would be available in intact vessels with average
dimensions under atmospheric pressure.
In conclusion, our work revealed the potential impact of cut-open
conduits on embolism resistance in leaf xylem, and we found that
embolism spreading during dehydration may happen at a less negative
water potential when there is a direct connection to a gas source. This
process could be minimised by hydraulic segmentation, with narrow and
short conduits slowing down gas diffusion due to their confined and/or
poorly interconnected nature.