Experiment 3: Is the Pneumatron underestimating embolism
resistance compared to the optical method?
The optical method and Pneumatron were simultaneously applied to the
same detached leaves for all six species. We were unable to obtain
Pneumatron measurements for B. pendula , because the gas volume
that could be extracted from the leaves was too small to meet the
sensitivity requirements of the tube volume, even if we reduced the
volume of the discharge tube to the minimum volume of 1.1 mL.
Vulnerability curves based on the two methods were obtained for five
species (Figure 4). The PEP12, PEP50 and
PEP88 values of detached leaves obtained with the
optical method in experiment 1 (Fig.1, Table S2) did not differ
significantly from those obtained in experiment 3 (Table S3). A
difference in embolism spreading from cut conduits to intact ones versus
spreading between intact conduits would especially be reflected in
PAD12 and PEP12 values. Indeed, average
PAD12 values based on the Pneumatron were lower than the
average PEP12 values based on the optical method for
four species, except for P. avium . The difference between
PAD12 and PEP12 was more than 0.6 MPa
for Q. petraea , F. sylvatica , and C. betulus ,
although these differences were not statistically significant. Also,
there was considerable variation among the samples tested (Fig. 5a),
with several samples showing a slightly more negative
PAD12 value than PEP12 for P.
avium and L. tulipifera .
Despite minor differences between PEP12 and
PAD12, the vulnerability curves based on the optical and
pneumatic method showed strikingly similar patterns for the five species
tested, with a strong correlation between PAD50 and
PEP50, and between PAD88 and
PEP88 (Fig. 5b, c). When intraspecific differences in
embolism resistance were found within a species, both methods matched
each other very well (Figure 4, 5, Figure S6). Although curves based on
the optical method had a relatively steeper slope compared to the
pneumatic curves for F. sylvatica , C. betulus and Q.
petraea , this difference was not significant. No significant difference
was found between PAD50 and PEP50, and
between PAD88 and PEP88. Nevertheless,P. avium showed a 0.67 MPa difference (P = 0.287) between
PEP50 and PAD50, and a 0.61MPa
difference (P = 0.454) between PEP88 and
PAD88 (Table S3).