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).