Figure 3. Cumulative embolized xylem area as a percentage of
total embolized area on the first cycle of dehydration (blue) in three
stems of Rhododendron hirsutum that were rehydrated at varying
degrees of embolized xylem area. The water potential at rehydration is
marked by a vertical dashed line. Cumulative embolized xylem area as a
percentage of the remaining un-embolized xylem area on the second cycle
of dehydration (red) is also shown. Temporal traces of water potential
and embolism accumulation through time can be found in Figure 2,
Supplementary Figure S1F and S1G. Similar traces for all other species
are found in Supplementary Figure S2.
Differences across species in the Ψ that triggered embolism during a
second cycle of dehydration had a considerable impact on whether
pre-existing embolism in the xylem altered apparentP50 . In many cases, pre-existing embolism caused
a minor increase in apparent embolism resistance, resulting in more
negative values of apparent P50(P 50r), which was P50calculated from data collected only from embolism events that occurred
on the second cycle of dehydration (Figure 4). We did note that in
angiosperms the higher the percentage of pre-existing embolism in the
xylem, the higher the percent change in apparentP 50, with P 50rparticularly differing from P50 changing once
40% of xylem was embolized (Figure 4). In conifers, the degree of
pre-existing embolism did not appear to alter apparentP50 as much as in angiosperm species (Figure 4).
In some conifer stems, with pre-existing embolism, apparentP 50 was slightly more negative, while in others
apparent P 50 was less negative or even unchanged,
irrespective of the degree of pre-existing embolism in the xylem
(Supplementary Figure S1).