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.