Large conduits typically embolise first, but are not more
vulnerable to embolism than narrow ones
Wide and long vessels in the midrib and secondary veins were found to
embolise before the high vein orders (3rd –
5th vein orders) in the three experiments conducted.
This pattern confirms various studies based on the optical method and
microCT observations (Klepsch et al. , 2018; Scoffoni et
al. , 2017; Brodribb et al. , 2016a). However, the observation of
local spreading of embolism in minor veins near cut vessels in
experiment 2, supports the hypothesis that embolism spreads from
pre-existing gas sources, which happened in minor veins prior to
embolism formation in large vessels of major veins (Fig. 3). This also
suggests that proximity to a gas source is the main driver for embolism
spreading, and not the conduit diameter per se.
Nevertheless, wide and long vessels are more likely to be connected to a
pre-existing embolism than narrow, short conduits, as discussed above.
It is also possible that large vessels may show a higher amount of
intervessel pit membrane area than narrow, short vessels. Although this
relationship has been examined with respect to the rare pit hypothesis
(Wheeler et al. , 2005; Hacke et al. , 2006), no support for
this idea was found in Acer (Lens et al. , 2011). Yet,
large and wide vessels are likely to embolise first, and this temporal
difference could be caused by their connectivity to a gas source, and
may not reflect any inherent difference in embolism resistance per se.
No alternative mechanism is known why wide conduits would be vulnerable
to embolism, since pit membrane thickness, which is strongly associated
with embolism resistance (Li et al. , 2016), was not related to
conduit diameter (Kotowska et al. , 2020; Wu et al. , 2020).