4.1 Phloem ontogeny and lianescence
During stem ontogeny, lianas that rely exclusively on twining for climbing, such as A. scandens , transition abruptly from self-supporting branches, known as searchers, to stiffer ones that grow in close contact with a solid support. In A. scandens , the photosynthetic, non-lignified searcher branches contain more phloem than xylem tissue, likely because the immature leaves have little demands of water, but a significant need of carbohydrates. This limited xylem implies that pith turgor might be the major contributor to the mechanical stability of searcher branches, as reported in other twining vines (Isnard & Silk, 2009). Morphologically, the tube-like primary sieve elements that dominated the vasculature of searchers suggested lack of lateral transport, but directional sap flow toward growing branch tips. Chemically, sieve tube walls of searcher stems were composed of a galacturonan-rich pectin, which has been described in a handful of species such as in leaves of Beta vulgaris (Torode et al., 2018), or in the seasonally renovated sieve tubes of Populus(Ray & Savage, 2020). These previous works strongly emphasized that the chemistry of the cell walls connect with mechanics of the sieve tube wall (Torode et al., 2018). Thus, this wall related epitope may contribute to the flexibility of searcher branches, and it may also be present in sieve tubes in the secondary phloem. Nevertheless, its presence in the primary sieve tubes of A. scandens demonstrates that this distinctive pectin is found in the sieve tubes of angiosperms with different life histories, growth habits, and phylogenetic backgrounds.
Despite the conservation of diameter in rigid twining stems, leaf area increased and reached full expansion. Basipetally, the phloem in the twining stems acquired a lobed topology, and the sieve tubes an increasing number of plate connections, features that suggest an increase in transport efficiency under torsion. In fact, the complex division patterns (i.e. tangential longitudinal) of the secondary phloem of A. scandens were evaluated in detail about 50 years ago (Srivastava, 1970), and differed from axial longitudinal divisions observed in other woody angiosperms, suggesting a correlation with the helical gyres of the lianoid growth habit. In line with these observations, we further noticed that, in areas of high rotation at the base of the stems, the sieve tube elements were shorter (Silk & Holbrook, 2005), reinforcing the idea that the vascular cambium activity is influenced by the mechanics of lianoid stems (Pace et al., 2015, 2018). How this mechanical (un)stability affects function has been studied for the lianoid xylem, which enhances hydraulic efficiency through more volumetric vessels despite the strong torsional pressures such as twisting and bending (Putz & Holbrook, 1992; Gentry, 1991; Speck & Rowe, 1999; Rowe et al., 2006). Our measurements revealed that sieve tube geometries are more conserved across species, and A. scandens displays radii of sieve elements (7-13 µm), which fall within parameters measured in the slender stems of Ipomoea nil (10-20 µm) (Knoblauch et al., 2016), or the trunks of canopy dominant trees (6-24 µm) (Liesche et al., 2017; Savage et al., 2017). The radii of the sieve tube elements increase basipetally at long distances, as observed in tall trees (Jensen et al., 2012, Savage et al., 2017). Strikingly though, the average sieve plate pore size of A. scandens (0.20µm radius) was smaller than in stems of other species, and varied little axially. Because pore size is the factor that affects phloem hydraulics the most, A. scandens has high phloem resistance in the stem, compared with other species (Liesche et al., 2017; Savage et al., 2017; Clerx et al., 2020; Barceló-Anguiano et al., 2021a), but within the parameters observed in other Austrobaileyales of the understory (Losada & Holbrook, 2019). To compensate for the small sieve plate pore size, the secondary phloem of A. scandens has an unusually high number of sieve plates along the tangential walls of phloem conduits. Compound sieve plates have been correlated with growth form in height across woody species that reach the forest crowns (Pace et al., 2015; Knoblauch et al., 2016; Liesche et al., 2017; Savage et al., 2017; Losada & Holbrook, 2019; Clerx et al., 2020; Barceló-Anguiano et al., 2021a,b). Reasons behind the particular phloem morphology of A. scandens(lobed arrangement in cross sections, tapered tubes with angled plates and different lengths) may reflect life history traits associated with the liana growth form, such as the imbalance of carbon allocation between the leaf canopy and stem tissues. Thus, a reduced number of below-canopy sink tissues and higher canopy sinks favor a more functional phloem aboveground, whereas in the understory, the mechanics of twining constrains the morpho-functional aspects of phloem (for example, the length of the sieve tubes).