1 INTRODUCTION
Lianas constitute about 40% of the woody individuals in tropical
forests and make an outsize contribution to forest productivity and
response to disturbance (Gerwing & Farias, 2000; Schnitzer et al.,
2012) as a result of their ability to elevate a dense leaf canopy tens
of meters aboveground with a minimal investment in self-support (Darwin,
1868; Baillaud, 1962). Anatomical features that allow lianas to meet the
evaporative demands of their leaves despite having slender stems include
large diameter xylem vessels, and thus greater hydraulic efficiency,
compared with co-occurring trees and shrubs (Carlquist, 1991; Ewers et
al., 1991; Isnard et al., 2003; Isnard & Silk, 2009; Wyka et al., 2013;
Chen et al., 2014; Pace et al., 2015, 2018; Chery et al., 2020). The
allometrical demands of the climbing habit might be expected to extend
to the phloem. Yet, the long-distance effectiveness of carbohydrate
transport remains poorly documented in lianas, due to the scarce
information on the intersection between the vascular structure of the
phloem and lianescence.
Previous anatomical descriptions of the stems of large-bodied vines
belonging to Loganiaceae, Malpighiaceae, Bignoniaceae, Fabaceae or
Sapindaceae emphasize novel topologies with distinctive cambia:
anomalous phloem architectures that evolved with the scandent habit and
represent traits associated with higher flexibility and resilience to
damage (Ewers et al., 1991; Fisher & Blanco, 2014; Moya et al., 2017;
Pace et al., 2011; 2015; 2018; Chery et al., 2020). Structural variation
of sieve tube elements in relation to axial transport, however, has only
been explored in the long and slender stems of Ipomoea nil(Knoblauch et al., 2016), which showed that while sieve tube geometry
varied little axially, sieve pores connecting conduits increased in
diameter from the top to the base of the stem, in agreement with the
pressure-flow hypothesis developed by Münch in 1930 (Knoblauch et al.,
2016). In other woody life forms such as trees and shrubs, axial scaling
of sieve tubes includes structural variation of phloem conduit diameter
and length, the size of pores, and the number of sieve plates, all
contributing to a more efficient transport at long distances (Liesche et
al., 2017; Savage et al., 2017; Losada & Holbrook, 2019; Clerx et al.,
2020; Barceló-Anguiano et al., 2021a,b).
Austrobaileya scandens is a large-bodied and long-lived liana
native to Queensland (Australia), which reaches more than 15m
aboveground by twining around trees as its only support (Bailey & Swamy
1949). A. scandens exhibits low photosynthetic rates and a slow
stomatal response to drops in environmental humidity (Feild et al.,
2003a,b; Feild & Arens, 2005, 2007; Feild & Wilson, 2012; Barral et
al., 2013). Anatomically, the stems of A. scandens display wide
xylem vessels (Carlquist, 2001) and a secondary phloem with extremely
angled compound sieve plates (Behnke, 1986). This sieve tube morphology,
which resembles the phloem of gymnosperms, drove early speculations on a
possible transitional tissue toward the ‘pipe-like´ sieve tubes of most
angiosperms (Bailey & Swamy, 1949; Srivastava, 1970; Behnke, 1986). An
alternative perspective is that angled plates compensate for small pore
diameters, or that the peculiar morphology of the secondary phloem ofAustrobaileya possibly relates to maintaining the functional
demands of the phloem as climbing stems undergo mechanical deformation.
To better understand vascular transport in large-bodied lianas, we
studied the phloem of leaves and stems, so far missing in this species.
The main goal of this study was to understand the structure of the
phloem in relation to 1) axial transport of carbohydrates from the sites
of photosynthesis through the slender stems; 2) mechanical demands of
climbing.