Figure 2. (A, B) Representative trace of the percentage of
embolized xylem area (blue) and stem water potential
(Ψstem) (black) plotted against time in a branch of the
angiosperm Rhododendron hirsutum . The vertical dashed lines
indicate the period between rehydration and the point at which the
Ψstem on the second dehydration had declined to the
Ψstem when the branch was first rehydrated. (B) The
relationship between the area (%) of pre-existing embolism at
rehydration and the relative area (%) of xylem embolised on the second
cycle of dehydration occurring at a more hydrated Ψ than when the branch
was rehydrated. An exponential relationship is plotted
(P<0.0001, R2=0.69). A mean (n = 3, ± SE) is
shown for T. californica . All traces used to assemble this
dataset are included in Supplementary Figure S1. Species abbreviations
are Tc T. californica ; Ar Agathis robusta ;Ts.c Tsuga canadensis ; Dw Drimys winteri ; Rh
R. hirsutum ; Iv Ilex verticillata ; Fr Ficus religiosa ;Ti.c Tilia cordata ; Lb Lindera benzoin.
When branches, regardless of whether they had xylem comprised of vessels
or only tracheids, were rehydrated at a Ψ more hydrated thanP 50, there were very few if any embolism events
observed on the second cycle of dehydration at a Ψ more hydrated than
when the stem was rehydrated (Figure 2B; Supplementary Figure S2). Once
xylem had experienced around 60% of area embolized then an increasing
percentage of the remaining xylem on the second cycle of dehydration was
observed to embolize at a Ψ that was more hydrated than when the stem
was rehydrated (Figure 2B; Figure 3). In three individual stems ofR. hirsutum that were rehydrated at different percentages of
embolized xylem area we found on the second cycle of dehydration that
there was little or no embolism formation (1.5% or 0% of total xylem
area) prior to stem dehydration to the Ψ at which rehydration occurred
when either 10.4% or 49.5% of total xylem area was embolized,
respectively (Figure 3). This contrasts with a branch that was
rehydrated when 81.7% of xylem area had embolized, in this branch on
the second cycle of dehydration embolism began to form at -0.56 MPa
(initial embolism events on the first dehydration occurred at a mean Ψ
of -0.96 ±0.05 (SE) MPa) and continued to accumulate linearly with
declining Ψ (with a slope of 3.7% remaining embolized xylem area
bar-1) such that when the stem had reached the Ψ at
which rehydration occurred more than 56% of the remaining xylem area
had embolized (equivalent to 10% of the total xylem area of the stem)
(Figure 3).