Vessel Size, Distribution, and Fiber Relative Area Differentiate
Self-Supporting and Lianescent Xylem
To characterize the
phenotypic changes in B. magnifica wood, we examined the xylem
anatomy of mature stems showing the self-supporting and lianescent
xylem. The careful analysis of
multiple images of each sectional plane showed that the self-supporting
xylem consisted of a matrix of septate fibers and rays, with vessels and
scanty paratracheal axial parenchyma produced solely by the fascicular
cambium, opposite the primary xylem poles (Fig. 3a, left). This
restriction on vessel production was sustained throughout the
self-supporting phase, conferring a radial arrangement to the vessels
opposite the protoxylem poles. The rays in the self-supporting xylem
were uniseriate and composed of upright and square cells (Fig. 3b,
left).
In contrast, the onset of the lianescent xylem was marked by the
simultaneous production of several large vessels throughout the cambium
width by both the fascicular and interfascicular cambium (Fig. 3a, white
arrows). These large vessels had a diameter of 111µm and were solitary
or associated with small vessels and formed a bimodal distribution of
vessel diameter classes (Fig. 3c). The lianescent xylem also had septate
fibers, but the rays were uni to biseriate and composed of predominantly
procumbent and square cells (Fig. 3b, right).
Quantitative analysis revealed significant differences between the two
phases (Table 1). Vessel relative area increased from 5.7% in the
self-supporting phase to 35.9% in the lianescent xylem. This increase
was mainly at the expense of fiber relative area, which decreased from
81.6% in self-supporting xylem to 54.7% in lianescent xylem. The
increase in vessel relative area was primarily driven by vessel size,
while vessel density remained constant. Consequently, the potential
specific conductivity of the xylem increased more than 20-fold, from
4.52 × 10-6 kg m-1Mpa-1 s-1 in self-supporting xylem
to 106.98 × 10-6 kg m-1Mpa-1 s-1 in lianescent xylem.
Intervessel-pit diameter and vessel grouping index, both related to
water conduction, also increased, while fibers in the lianescent xylem
were shorter, wider, and had thicker cell walls compared to the
self-supporting xylem. These changes in cell dimensions resulted in an
overall reduction in cell density (number of cells per area).