Abstract
The changes in cellular structures that occur in cambial cell derivatives during xylogenesis
were examined in Populus trichocarpa Torr et Gray. During dormancy, the cells of the vascular cambium are characterised
by dense cytoplasm, many small vacuoles and lipid bodies. During cambial activation,
cambial cells are highly vacuolated, the cytoplasm is rich in organelles and the nucleus
contains distinctly enlarged nucleoli. The plasma membrane forms vesicle-filled invaginations
which mediate uptake of vesicular material into the vacuole. The mitotic patterns
in dividing fusiform cells are fragmentary due to their strong vacuolisation. During
cell enlargement, cambial cell derivatives remain strongly vacuolated and cytoplasmic
structures are similar to active fusiform cells. From the beginning of secondary cell
wall formation many changes in cytoplasmic structures occur in newly-formed fibres
and vessels. In fibres, the cytoplasm is characterised by components of secondary
cell wall synthesis, as indicated by increased amounts of endoplasmic reticulum, vesicle-producing
dictyosomes and microtubules. In contrast, vessels show a more or less distinct occurrence
of these components and remain more strongly vacuolated than fibres. Similar to cambial
cells, a distinct flow of vesicular material into the vacuole through invaginations
of the plasma membrane is apparent in fibres, as well as in vessels. After completion
of the secondary cell walls, the loss of tonoplast integrity causes the collapse of
the vacuole and initiates cell death in vessels and fibres. In vessels the tonoplast
exhibits unusually strong staining prior to the collapse of the vacuole, indicating
subsequent cell death. Overall, our results indicate an important role for the vacuole
in the xylogen differentiation of cambial derivatives.
Key words
Cambial cell derivatives - ultrastructure - xylem differentiation - poplar
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M. Arend
Technische Universität München
Fachgebiet Angewandte Holzbiologie
Winzererstaße 45
80797 München
Germany
Email: arend@holz.forst.tu-muenchen.de
Section Editor: H. Rennenberg
