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.