Embolism spreading depends on pre-existing embolism as gas
source
As shown in experiment 1 and 2, embolism spreading happens largely from
one embolised conduit to a neighbouring one, which is in line with the
air-seeding hypothesis (Zimmermann, 1983; Sperry and Tyree, 1988).
Embolism formation appears to be unlikely if a conduit is not connected
to a pre-existing embolism. Novel embolism formation has been observed
based on microCT (Brodersen et al. , 2013, Choat et al. ,
2015, 2016), and embolism formation in seemingly isolated conduits could
occasionally be observed in our experiments. However, the rather
two-dimensional view associated with the optical method and its limited
resolution to accurately detect the narrow vessel ends (Oskolski and
Jansen, 2009), did not allow us to confirm that these conduits were
completely disconnected from neighbouring gas sources.
While proximity between a studied xylem area and cut conduits seems to
be important, the speed of embolism spreading over a certain distance
also depends on the vessel dimensions. Wide and long vessels would
indeed show a faster propagation of embolism over a given distance than
narrow, short vessels if embolism spreading are mainly single-vessel
events (Johnson et al. , 2020). Spreading of embolism would
especially be reduced by xylem areas with hydraulic segmentation, making
these xylem patches seemingly more embolism resistant. The four species
that showed a reduced embolism resistance in detached leaves as compared
to leaves attached to branches, have open vessels running directly from
the base of the petiole into the midrib (Table 1, Figure 2). Since the
maximum vessel length in petioles of L. tulipifera and B.
pendula were shorter than the petiole length (Table 1), both species
showed relatively small differences in embolism resistance between
detached leaves and leaves attached to a stem segment (Fig. 1c, e).
Why does hydraulic segmentation reduce embolism spreading? Firstly, it
is likely that hydraulic bottlenecks show locally highly reduced conduit
dimensions, especially with respect to conduit length and width, with a
high number of interconduit end walls over a short stretch of xylem
tissue. Conduit end walls have been suggested to hold up embolism
spreading at least temporarily, with pit membranes functioning as safety
valves and preventing further spreading of embolism due to their tiny
pores (Zhang et al. , 2017, 2020; Kaack et al. , 2019;
Johnson et al. , 2020). Moreover, narrow and short tracheids or
fibriform vessels may be more confined than long and wide vessels, with
a small interconduit pit membrane area for air entry. A recently
embolised intact vessel is not immediately filled with gas under
atmospheric pressure, but eventually achieves Henry’s law equilibrium
based on the speed of gas diffusion. While cut-open vessels are
immediately filled with air and reach atmospheric pressure immediately,
intact vessels that embolise become filled with a mixture of water
vapour and air. It has been modelled that it takes from 20 min to
several hours to obtain atmospheric pressure in embolised, intact
vessels, which depends on the distance to the nearest gas phase, and the
interconduit pit membrane area for gas diffusion (Wang et al. ,
2015a, b). Although gas diffusion happens also across conduit cell
walls, the micropores (< 2 nm) in hydrated walls are much
smaller than the 5 to 50 nm dimensions of pit membrane pores (Donaldsonet al. , 2019; Kaack et al. , 2019). Therefore, it is
reasonable to assume that gas diffusion across hydrated pit membranes is
much faster than across the various layers of cell walls. Moreover,
end-wall resistivity of conduits has been suggested to be proportional
to lumen resistance (Hacke et al. , 2006; Sperry et al. ,
2005), while conductance of gas increases to the 4thpower with conduit diameter or pore diameter according to
Hagen-Poiseuille’s equation.
Where does the gas come from to induce embolism in the first conduits?
Since vessels and tracheids do not show pits with non-conductive fibres
(Sano et al. , 2011), it is unlikely that gas diffusion from these
cells or intercellular spaces will contribute to embolism formation of
conduits. It is possible that there is almost always an embolised
conduit available, perhaps in primary xylem or in older xylem from an
older growth ring. This would be an obvious gas source if functional,
sap-filled conduits show any direct connection with these embolised
conduits via bordered pits. However, these pre-existing gas sources may
not be available, or may not be connected to the hydraulic network of
the current year’s sapwood due to segmentation. When comparing embolism
resistance of leaves attached to long stem segments with embolism
resistance of stem xylem based on previous papers (Klepsch et
al. , 2018; Zhang et al. , 2018), all six species studied showed
that leaf xylem was between 0.5 and > 1 MPa more embolism
resistant than stem xylem. Our result of B. pendula was
consistent with Klepsch et al. (2018), with leaf xylem being more
resistant than stem xylem. Other angiosperms species, however, showed
that stem xylem was either more embolism resistant, or equally resistant
than leaf xylem (Chen et al. , 2009; Zhu et al. , 2016;
Skelton et al. , 2018; Losso et al. , 2019). This
discrepancy seems to suggest that sufficient caution is needed to
directly compare absolute values of embolism resistance between
different methods and organs.