Figure 1. The relationship between cumulative embolised xylem
area in stems and stem water potential (Ψstem) during a
cycle of dehydration, rehydration and dehydration in the angiospermIlex verticillata (A and B) and the gymnosperm Agathis
robusta (C and D), with vessel and tracheid-based xylem, respectively.
In A and C the solid red lines indicate Ψstem at which
50% of the xylem area was embolized (P 50) on the
initial dehydration, and the Ψstem at which 50% of the
remaining stem xylem area was embolized (P 50r) on
the second dehydration. The dashed red line represents the beginning of
second period of dehydration. The solid blue line represents the
embolized area when rehydration occurred. In B and D cumulative
embolized xylem area as a percentage of total embolized area (blue) on
the first cycle of dehydration is shown alongside the cumulative
embolized xylem area as a percentage of remaining un-embolized xylem
area on the second cycle of dehydration (red). Horizontal red lines
indicate when 50% of the total or remaining xylem area was embolized.
Temporal traces of water potential and embolism accumulation through
time can be found in Supplementary Figures S1A and S1B.
On the second period of dehydration, embolism occurred in the remaining
conduits that had not embolised prior to rehydration. Differences were
observed in the Ψ at which these embolism events first formed on the
second dehydration that influenced the apparent embolism resistance of
the remaining water-filled xylem conduits (Supplementary Figure S1). In
most species embolism events on the second dehydration often did not
form until Ψ had declined to values similar to those measured when the
stem was rehydrated (Figure 1B; Supplementary Figure S2). In I.
verticillata the impact of a lack of embolism formation at relatively
high Ψ compared to when the first conduits embolized when initially
dehydrated, meant that apparent P50(P50r ) was 0.69 MPa lower in the stem with 50%
of xylem embolized, than P50 in the stem when no
xylem was embolized (Figure 1A and B). In contrast in the conifer
species assessed, embolism was often observed to form at the Ψ at which
embolism initially started to occur on the first dehydration (Figure 1C
and D; Supplementary Figure S1). In A. robusta this resulted in
an apparent P50r that was only 0.06 MPa lower in
the stem with 50% of xylem embolized, than P50in the stem with no pre-exisiting embolism (Figure 1C and D).
Differences between species in the nature of xylem embolism formation
during rehydration and on the second cycle of dehydration were also
apparent when cumulative embolism observations were plotted against time
(Figure 2). In a representative trace from a stem of the angiosperm
species Rhododendron hirsutum with vessel-based xylem, embolism
first started to occur once Ψ reached -1.0 MPa, and progressively
continued until -1.54 MPa, when approximately 50% of the total xylem
area was embolized (Figure 2A). After rehydration, which occurred at
-1.54 MPa, there was a complete and fast cessation of embolism
accumulation (Figure 2A). During the second cycle of dehydration, new
embolism events were only observed to form in the xylem after Ψ had
declined to -1.54 MPa, which occurred 10 h after rehydration (Figure
2A). There was a similar slope in the decline of water potential through
time on the first and second dehydration, however, once embolism began
to form on the second dehydration the slope of water potential decline
over time declined.