Embolism spreading may not rely on a fixed pressure difference
threshold, but on pressure-driven diffusion
The observation of embolism spreading under highly variable xylem water
potentials in experiment 1 and 2 indicates that embolism formation may
not always rely on a certain threshold of the pressure difference
between a functional and embolised conduit, as frequently assumed based
on the air-seeding hypothesis, and quantitatively estimated by the
Young-Laplace equation (Sperry and Tyree, 1988; Choat et al. ,
2008). Even if some of our xylem water potential measurements may not be
fully accurate due to a pressure gradient heterogeneity (Bouda et
al. , 2019), the > 1 MPa difference in
PEP50 values between detached leaves and leaves
connected to stems provides solid evidence that embolism formation in
xylem tissue from the same organ of a species may occur under different
xylem water potentials. This finding is not entirely new, and in line
with earlier differences in embolism resistance between intact plants
and cut plants (Choat et al. , 2010; Torres-Ruiz et al. ,
2015; Lamarque et al. , 2018). For instance, a ca. 4 MPa
difference in P50 was found for Laurus nobilisbased on microCT observation of cut branches and intact seedlings
(Nardini et al. , 2017; Lamarque et al. , 2018). Similar to
our results, Skelton et al. (2018) compared the vulnerability
curves of cut branches and intact plants based on the optical method forQuercus wislizenii , and found a 1.5 MPa difference in
P50 between leaves attached to a long, cut branch, and
leaves from an intact plant. The finding that cut plant material can be
more vulnerable to embolism spreading than intact plants raises concerns
about embolism resistance measurements of plant material samples with
pre-existing embolism, the possible induction of embolism due to
cutting, and the application of the bench dehydration method on cut
plant material (Sperry and Tyree, 1988).
If embolism spreading for given species may not rely on a certain
pressure threshold, which mechanism does then trigger embolism? We
believe that this is a highly important question in our understanding of
water transport, even though this question cannot be fully answered
based on the available evidence. We speculate that gas diffusion across
interconduit pit membranes plays a role in determining the amount of gas
dissolved in xylem sap, which may affect embolism nucleation. Although
mass flow is theoretically 105 times faster than
diffusion, gas diffusion in xylem is overall much faster and more common
than mass flow. The main reason seems to be that gas diffusion takes
place continuously over very large areas, while mass flow requires gas
movement through multi-layered, tiny pore constrictions of mesoporous
pit membranes (Kaack et al. , 2020). However, the continuous
nature of gas diffusion and the high amounts of gas (CO2and O2) surrounding conduits (Spicer and Holbrook, 2005;
Teskey et al. , 2008) do not mean that gas concentration in xylem
sap is always under equilibrium with gas in embolised conduits, as has
frequently been assumed (Hammel 1967; Yang and Tyree, 1992; Wheeleret al. , 2013; Schenk et al. , 2016). Gas solubility of
xylem sap is affected by pressure and temperature (Mercury et
al. , 2003; Schenk et al. , 2016; Lidon et al. , 2018). Due
to a nearly constant atmospheric pressure in a cut-open vessel, but a
considerably variable liquid water pressure in sap-filled conduits, the
driving force for gas diffusion is the unbalanced, anisobaric situation
between embolised conduits and functional conduits. We ignore here the
effect of temperature on gas solubility of xylem sap as all experiments
were conducted in an air-conditioned lab under similar temperature
(Schenk et al. , 2016). Moreover, oversolubility of gas may occur
in nanoporous cell walls and mesoporous pit membranes, as is known for
nanoconfined environments in general (Pera-Titus et al. , 2009; Hoet al. , 2015; Coasne & Farrusseng, 2019), which could contribute
to concentration gradients of gas dissolved in xylem sap.
A recently embolised intact vessel is not immediately filled with gas
under atmospheric pressure, but eventually achieves equilibrium of its
gas pressure with the atmosphere, depending on how fast gas is attracted
from surrounding gas sources via diffusion. While cut-open vessels are
immediately filled with air and reach atmospheric pressure immediately,
intact vessels that embolise are initially filled with water vapour (ca.
2.4 kPa). It has been modelled that gas diffusion takes from 20 min to
several hours to obtain atmospheric pressure (ca. 101.3 kPa) in
embolised, intact vessels, which may depend on the distance to the
nearest gas phase, and the interconduit pit membrane area for gas
diffusion (Yang & Tyree, 1992; Wang et al. , 2015a, b). Although
gas diffusion happens across conduit cell walls and pit membranes, the
micropores (< 2 nm) in hydrated walls are much smaller than
the 5 to 50 nm dimensions of pit membrane pores (Donaldson et
al. , 2019; Kaack et al. , 2019). Therefore, it is reasonable to
assume that gas diffusion across hydrated, 200 to 600 nm thick pit
membranes is much faster than across the much thicker layers of
secondary cell walls (Yang et al. , submitted). Indeed, axial gas
diffusion in wood is found to be about one to two orders of magnitude
larger than radial diffusion (Sorz & Hietz, 2006). Moreover, end-wall
resistivity of conduits has been suggested to be proportional to lumen
resistivity (Sperry et al. , 2005; Hacke et al. , 2006), and
conductance of gas increases to the 4th power with
conduit diameter or pore diameter according to Hagen-Poiseuille’s
equation.
Further research is clearly needed to investigate how gas diffusion may
contribute to the very high gas solubility of xylem sap (Schenk et
al. , 2016), whether oversolubility occurs due to nanoconfined spaces
such as pit membranes and cell walls (Pera-Titus et al. , 2009; Hoet al. , 2015; Coasne & Farrusseng, 2019), how gas-xylem sap
interfaces are affected by the dynamic surface tension of xylem sap
lipids (Yang et al. , 2020), and how surfactant-coated nanobubbles
may contribute to the gas concentration of xylem sap and embolism
formation (Schenk et al. , 2015, 2017; Jansen et al. , 2018;
Park et al. , 2019).