3.4 Phloem hydraulic resistance from leaves to stems ofAustrobaileya scandens
Anatomical data (average sieve tube radii, and pore radii/length at each
location along the transport pathway in both leaves and stems) was used
to estimate phloem hydraulic resistance, supposing a continuous
pipe-like cylindrical tubing system formed by sieve elements connected
in series, as modelled previously for vines and trees (Knoblauch et al.,
2016; Savage et al., 2017). The Hagen-Poiseuille equation was the
reference to understand the axial resistance of the sieve tube. Due to
the difficulty of measuring the actual length of the twisted stems, we
calculated the total resistance per tube length (Pa s
m-4) at the defined axial positions along the
transport path from leaves to the base of the stem. The poor
preservation the thinner twining stems allowed only a few measurements
of pore size, which revealed an average radius of 0.19µm, but this was
quite similar to the pore radius at the base of the stem (0.20µm), where
preservation was much better and resulted in n=600 pores measured. The
following assumptions were made for calculations of the hydraulic
resistance: (1) sieve areas and pore density were equal to the midrib
value for higher vein orders; (2) pore size at the tip of the stems were
similar to those measured at the vine base. The latter assumption is
very conservative, and likely underestimates the resistance of the upper
stem, given that previous reports showed smaller pore sizes at the tips
of vine stems compared with their base (see Knoblauch et al., 2016).
Sieve tube resistance varied about one and a half orders of magnitude
from the branch tips to the base of the stems. Compared with trees of
similar height, the top of the vine displayed similar sieve tube
resistance, but this resistance dropped more moderately than in trees,
resulting in two orders of magnitude higher resistance of A.
scandens vines at ground level (Savage et al., 2017; Losada &
Holbrook, 2019; Clerx et al., 2020). The small pore radii of sieve
plates were a major factor contributing to this high resistance (Figure
8), although compensated by an increasing number of sieve plates from
top to bottom, thus overcoming resistance penalties associated with
length. Monotonic decrease of axial resistance due to axial conduit
structural variation encounters an exception at the base of the vine,
where conduits were shorter.