Ultrastructural changes in cambial cell derivatives during xylem differentiation in poplar

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.