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 ( = 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).