Experiment 1: Does embolism spreading depend on the proximity to
cut conduits?
The shape of the vulnerability curves obtained was consistently
sigmoidal for the three sample types, i.e. detached leaves, leaves
attached to a short stem segment, and leaves attached to a long stem
segment (Figure 1). Embolism expansion in leaf veins started typically
in major veins and proceeded to minor veins (Figure S4). We did not see
different patterns in the progression of embolism formation among
detached leaves and leaves attached to a short or long stem segment.
There was considerable variation in the PEP12,
PEP50 and PEP88 values among the three
types of samples for several species. Comparison of detached leaves with
leaves attached to short stem segments showed a significant difference
(P < 0.05) in PEP50 for C.
betulus , F. sylvatica , P. avium and Q. petraea(Figure 1, Table S2). Detached leaves of these species showed a ca. 1.5
MPa less negative PEP50 value compared to
PEP50 values of leaves on short stem segments. A minor
difference in PEP50 with no significant difference was
obtained for L. tulipifera and B. pendula (Figure 1, Table
S2).
A positive, exponential correlation (R² = 0.52, P< 0.05) was found for the shift in embolism resistance between
detached leaves and leaves attached to a short stem segment, and the
segmentation index of leaf xylem (Figure 2). Leaves with a segmentation
index > 1 were strongly affected by the cut-open vessels at
the petiole end, resulting in a shift in PEP12,
PEP50 and PEP88 of 1MPa or more between
detached leaves and leaves attached to stem segments. L.
tulipifera and B. pendula , which had all vessels ending within
their petiole, were clearly less affected by the proximity to cut xylem
conduits compared to the four other species with vessels running from
the petiole end into the midrib.
A significant difference in PEP12 and
PEP50 was also found between leaves attached to a short
branch, and those on a long branch for F. sylvatica . No
significant difference in xylem embolism resistance was found between
leaves on a short stem segment and leaves on a long stem piece for the
other species studied, except for PEP88 values ofB. pendula and P. avium . Any similarity or dissimilarity
in PEP50 between the three types of samples was mostly
reflected in PEP12 and PEP88 (Figure 1,
Table S2).