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.