4.3 Contrasting vascular strategies between leaves and stems of lianas in the understory
Our measurements, although limited in number, suggest that the velocity of the phloem in A. scandens is one order of magnitude slower than Ipomoea and in trees (Windt et al., 2006; Babst et al., 2013; Knoblauch et al., 2016), but similar to the rates observed inIllicium parviflorum (Losada & Holbrook, 2019). While the radii of the phloem conduits were similar in petioles and searcher branches, the hydraulic resistance from leaves to stems followed a continuum, differing by three orders of magnitude from the minor veins to the base of the plants. Yet, this is a smaller difference than that reported between the top and the bottom of tree stems of comparable heights (Savage et al., 2017). Our calculations, which assume a constant viscosity of 1.7 mPa s-1, would not allow transport at long distances with faster velocities. For example, the pressure required to transport the sap 3 m from the leaves would be 1.4 MPa, suggesting that longer distances could impair transport. In conclusion, with constant viscosity and velocity, the export of carbohydrates from the leaves is facilitated by the geometry of the phloem, but encounter architectural limitations in the stems (i.e. tiny pores). What this may imply is that the export of photoassimilates may easily be redirected toward the continuously growing canopy. This is possible because the stem girth is maintained constant for long distances, but enlarges typically at the base of the vine. Additionally, the stems of A. scandens are photosynthetic along their length, pointing to the possibility of self-sustaining, at least partially, the limited secondary growth, and root elongation.
The vesselless leaves of A. scandens were previously suggested as having a simpler anatomy than other angiosperms, such as the absence of palisade parenchyma, low stomatal density with slow responses to VPD changes (Feild et al., 2003), which correlated with a physiology associated to the understory environment (Brodribb & Feild, 2010). Similarly, woody members of the ancestral ANA grade, such asAmborella trichopoda , is vessel free and lacks reaction wood. (Feild et al., 2005; 2012). Altogether, these results that include a strong hydraulic segmentation of the xylem, and a high resistance of phloem to sap transport, correlate with the slow growth rate of this species in the understory conditions (Feild et al., 2012).
Exploring the phloem of extant members of the ancestral angiosperm grade ANA (Amborellales, Nymphaeales, Austrobaileyales) is particularly relevant, given that fossils, when available, rarely preserve this tissue. Thus, living members allow for the inference of the varied hydraulic solutions evolved -and perhaps maintained- by angiosperms during their initial radiation. A. scandens , the only extant member of the Austrobaileyaceae family, itself one of the three lineages composing the ANA grade, a sister grade to all flowering plants (Mathews & Donoghue, 1999; Parkinson et al., 1999; Qiu et al., 2000; Soltis et al., 1999, 2018), has been historically used to speculate on the primitive growth habits of the first angiosperms. In particular, one of the critical questions is when and how flowering plants reached the canopy (Judd et al., 2018). Our work provides the first empirical interpretation of long distance transport in woody lianas with simple body plans, and the constraints associated with their life histories. Woody lianas, semi climbers and shrubs (rarely trees) dominate the extant forms of the earliest angiosperm lineages Amborellales and Austrobaileyales. Woodiness, considered the symplesiomorphic condition of angiosperms as a whole, was likely required to colonize the vertical niche during the Cretaceous, previously dominated through millions of years by gymnosperms. The question remains as to whether the great anatomical diversity of climbers, typically considered as highly derived, could have been the ancestral condition of angiosperms.