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).