4.2 Vascular scaling in leaves of Austrobaileyales and other
angiosperms
Within leaves of A. scandens , the geometry of vascular elements
(tracheids and sieve tube elements) vary across vein orders.
Hierarchical scaling of the xylem has been documented in the reticulate
leaves of angiosperms, suggesting that the efficiency of water
distribution across the continuous xylem conduits follows laws of energy
conservation, such as Murray´s law (Murray, 1926; McCulloh et al., 2003;
Carvalho et al., 2017a, Scoffoni et al., 2017). In contrast, the scaling
of sieve tube elements in leaf veins of woody plants remains poorly
characterized. Recent work in leaves with different branching patterns,
such as the dichotomously branched veins of Ginkgo (Carvalho et
al., 2017b), or reticulate-veined leaves of Populus andIllicium (Carvalho et al., 2017a; Losada & Holbrook, 2019),
strongly suggest universal variation of the geometry of phloem conduits
across leaf vein orders. Yet, an unexplored feature in the leaves of
angiosperms is the size of pores connecting sieve tubes in the leaves.
Our work provides evidence that pore sizes vary in accordance with sieve
tube dimensions, from 0.08µm in minor veins to 0.12µm in the petiole. As
a result, variation of both the geometry of tubes and size of pores
enhance bulk export of photoassimilates toward the petiole under the
pressure-flow predictions (Münch, 1930).
We further report a 1:1.25 xylem to phloem ratio of areas of the major
vein in Austrobaileya leaves, balanced in favor of the phloem
compared to the general range of 1:4 to 1:10 reported in leaves of
deciduous trees (Artschwager, 1926; Waisel et al., 1966). Additionally,
the isodiametric sieve tube elements in the midrib linearly increased in
number toward the petiole. The dimensions of sieve elements in the major
leaf veins remains poorly explored in angiosperms, but they are well
documented in the single veined needles of conifers (Ronellenfitsch et
al., 2015), in which photo assimilate export is favored by the
increasing number of isodiametric phloem conduits from the tip to the
base of the needles. This suggests a convergent strategy between conifer
needles and A. scandens midrib, but whether this is
well-maintained in other angiosperm leaves, needs further testing.
Furthermore, the isodiametric tracheids of the midrib linearly increased
in number toward the petiole, supporting predictions on the uniformity
of xylem conduit diameter within the same vein order (McCulloh et al.,
2003, 2009; Gleason et al., 2018). Among the scarce studies documenting
xylem allometry in single veins, variation in conduit diameter along the
midrib has been studied in Fraxinus (Petit et al., 2016) andAcer (Lechthaler et al., 2019). The shortening of tracheids
toward the petiole of A. scandens suggests that conductivity of
water uptake by leaves is lower, in sharp contrast with the highly
conductive stems, pointing to hydraulic segmentation of the xylem
between the stems and the leaves. In contrast, the sieve tube elements
of the petiole reduced their length, but not their diameter, thus
maintaining a stable transport capacity. We previously showed a similar
trait in the leaves of Illicium parviflorum (Losada & Holbrook,
2019; Carvalho et al., 2018), a pattern confirmed in leaves of five tree
species: Acer saccharum , Liriodendron tulipifera ,Catalpa speciosa , Liquidambar styraciflua , andQuercus rubra (unpublished).
The special characteristics of the petioles have been previously put
forward in a number of species such as in the genera Beta (Geiger
et al, 1969), Cyclamen (Grimm et al., 1997), orPelargonium (Ray & Jones, 2018), suggesting architectural
plasticity that is uncoupled from the leaf lamina. Strikingly, we found
that the petiole of A. scandens is fiberless, in sharp contrast
with the massive presence of perivascular fibers in the leaf lamina.
Fiberless petioles were further observed in Illicium parviflorum(Losada & Holbrook, 2019), and suggest that these short petioles
require flexibility, especially in understory plants such as the
majority of members of the Austrobaileyales, which need to
orientate their leaves toward sun flecks. In addition, the petioles ofA. scandens may aid with twining, as previously suggested (Feild
et al., 2003b), in agreement with previous evidence of a pivotal role of
petiole reorientation in generating the squeezing force that stabilize
twining stems (Isnard et al., 2009).