Experiment 1: Does embolism spreading depend on the proximity to cut conduits?
The dehydration time for each leaf or branch usually took 10 to 48 hours. Linear regressions were fitted based on the water potential in the period that corresponded to 200 to 600 minutes after dehydration was started. The slope of this fitting characterised the speed of the dehydration process (Table S3). L. tulipifera and B. pendula were the slowest dehydrating species, with a xylem water potential drop of. 0.0012 and 0.0009 MPa min-1 for detached leaves, respectively, and a similar slope of the xylem water potential in leaves attached to a short branch for these species.Q. petraea showed a relatively high speed of dehydration of -0.0038 MPa min-1 for detached leaves and leaves attached to a short branch. Moreover, the dehydration speed of detached leaves was much faster than leaves attached to a long branch forF. sylvatica , P. avium , and C. betulus (Table S3).
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 S3). 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 S1). 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 S1).
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 P12, P50 and P88 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 of 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 S1).