Seasonal changes in the ultrastructure of the vascular cambium of Robinia pseudoacacia

The ultrastructure of the vascular cambium of Robinia pseudoacacia L. was examined in trunk tissues collected over a 2 1/2 year period. During dormancy, fusiform cells are densely cytoplasmic with many small vacuoles and centrally located nuclei. Mitochondria are round to oval in sectional view. The plastids are variable in shape, have few internal membranes, and generally lack starch grains. The plasmalemma is smooth in outline. Proteinaceous material occurs in the vacuoles and many lipid droplets are scattered throughout the ground substance. Smooth tubular ER, often highly dilated, predominates, but short segments of rough ER are also present. Abundant free ribosomes are evenly distributed throughout the ground substance and the dictyosomes are inactive. Microtubules are parietal and have various orientations. During reactivation, the plasmalemma becomes irregular in outline and begins to form invaginations. Concurrently, the proteinaceous material disappears, the vacuoles begin to fuse, polysomes appear, and the dictyosomes begin to produce vesicles. During the period of cambial activity, fusiform cells are highly vacuolate, and the nuclei are centrally located. The mitochondria are round, oval, or elongate. Now the plastids contain phytoferritin, starch grains, or both. Many large invaginations of the plasmalemma intrude into the vacuole, pushing the tonoplast inward and pinching off into the vacuole, which lacks proteinaceous material. Lipid droplets are scarce. Most ER is rough, and ribosomes are generally aggregated as polysomes. Dictyosomes are actively producing vesicles. During the transition to dormancy, the fusiform cells gradually assume the appearance typical of the dormant cambium.