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.