In the absence of considerable embolism individual conduits have
specific thresholds for embolism resistance
Our data suggest that in the absence of considerable embolism in
neighbouring xylem conduits, individual conduits have a relatively
unique, and highly conserved threshold Ψ at which embolism will occur.
This threshold, however, is not sustained when there is a high
proportion of embolized conduits in the xylem (more than 60%), an
observation which supports the gas availability hypothesis proposed by
Guan et al. (2021). We suggest that early during a drought, pit
membranes provide a sufficient barrier to prevent embolism spread from
gas-filled to neighbouring water-filled conduits, yet once more than
60% of conduits are gas-filled, diffusion of gas into water-filled
conduits can be facilitated and occurs at relatively minor pressure
differences across conduits.
Observations of variation in embolism resistance across xylem conduits
have been reported by microCT imaging (Knipfer et al. 2015; Choat et al.
2015b; Torres-Ruiz et al. 2016; Jacobsen et al. 2019). A number of key
anatomical traits vary across populations of xylem conduits that are
associated with embolism resistance, including
(t/b )3 and vessel diameter, although we have no
mechanistic explanation of how these traits affect embolism spreading
(Hacke et al. 2001; Blackman et al. 2010; Jacobsen et al. 2019; Scoffoni
et al. 2017b). In addition to anatomical differences that might set this
variation in inter-conduit embolism resistance, variation across age
class of xylem might also account for some of the spread in embolism
resistance thresholds between conduits, with older xylem in Vitisbeing more vulnerable to embolism formation during drought (Brodersen et
al. 2013), or more resistant within a single growing season (Sorek et
al. 2021).
In angiosperms, pore constrictions in multi-layered pit membranes and/or
a relatively high degree of isolation within the hydraulic conduit
network provides added protection from the spreading of pre-existing
embolism into water-filled conduits during drought (Johnson et al. 2020;
Schenk et al. 2008; Avila et al. 2021). The narrow size of pore
constrictions (< 50 nm) and the highly variable pore size
dimensions do not allow mass flow of gas from an embolised to a
water-filled conduit under negative pressure (Yang et al. 2020; Kaack et
al. 2019; Kaack et al. 2021). Surfactants coating the cellulose of pit
membranes are believed to promote gas entry from a neighbouring
embolised vessel into a water-filled conduit, but at the same time
mitigate embolism spreading (Schenk et al. 2021). In addition to
providing a physical barrier for the spreading of gas between an
embolized and water-filled conduit, pit membranes may provide a
short-term buffer to the further spreading of embolism, particularly
when there are small numbers of embolized conduits. Upon embolism
formation the water vapour in a conduit is at a negative pressure, and
modelling suggests that it may take between 20 min and 10 h for this
negative gas pressure to equilibrate with atmospheric pressure, during
which time the embolized conduit can draw gas slowly across cell walls
and rapidly across pit membranes from neighbouring gas filled, but also
water-filled conduits, potentially buffering water-filled conduits from
embolism (Wang et al 2015). Our experiments provide evidence that pit
membranes can act as safety valves to delay the spread of embolism
between neighbouring gas- and water-filled conduits.
Upon rehydration we did not observe conduit refilling (refilling of
conduits have been observed using the optical method in the hydroids of
a moss (Brodribb et al. 2020a)). Refilling of xylem on excised stem
rehydration has been reported to occur (Trifilò et al. 2014), although
the validity of these observations has been challenged (Lamarque et al.
2018). We did however observe a small degree of embolism formation
persisting in a few species after rehydration (Figure 1B). This lag time
in embolism cessation may be due to a slower, heterogenous reduction in
the pressure difference between conduits in the area of xylem that we
were observing with the optical method (Bouda et al. 2019), a phenomenon
exacerbated by the presence of widespread embolism and associated
decline in hydraulic conductivity. Furthermore, there may be a temporal
component to embolism resistance, such that the longer a conduit
experiences a negative Ψ the more likely the chance of changes in water
vapour and gas concentration in embolised conduits, or gas dissolved in
xylem sap, which may trigger embolism spread (Guan et al. 2021, Kaack et
al. 2021).
While embolism formation on rehydration was observed in all species,
there appeared to be a tendency for more embolism formation after
rehydration in conifer species (Supplementary Figure S1). To test
whether pit membrane anatomy or conduit anatomy might explain why in
some conifer species there was an exacerbated embolism spreading after
rehydration we included the angiosperm species D. winteri in our
sampling, a vessel-less species with homogeneous pit membranes, as
opposed to torus-margo pit membranes (Zhang et al. 2020). Unlike some
conifers, the formation of embolism after rehydration in D.
winteri was limited (Supplementary Figure S1C), suggesting that even
though this species has only tracheids, an altered conformation caused
by embolism is able to create a highly gas impermeable structure that
protects the neighbouring conduits from air invasion (Zhang et al.
2020).
Other factors might also provide an explanation for the greater absence
of embolism occurrence in angiosperm xylem after rehydration and on a
second cycle of dehydration until reaching the Ψ at which the branch was
rehydrated, including the increased separation of conduits imbedded in a
matrix of non-conductive xylem tissue, such as fibres and parenchyma
that might offer a physical barrier for rapid air propagation through
the xylem (Johnson et al. 2020; Avila et al. 2021), particularly
compared to the tracheid based xylem of conifers, which is largely
homogeneous and comprised of closely packed tracheids. The close
proximity of tracheids with tracheid tips slightly bent and overlapping
multiple neighbouring tracheids may facilitate a more rapid spread of
embolism on a second dehydration cycle in conifers (Torres-Ruiz et al.
2016; Choat et al. 2015b), while the 3D reconstruction of vessel
networks deserves more attention for angiosperms (Wason et al. 2021).