A SEASONAL CYCLE OF CELL-WALL STRUCTURE IS ACCOMPANIED BY A CYCLICAL REARRANGEMENT OF CORTICAL MICROTUBULES IN FUSIFORM CAMBIAL CELLS WITHIN TAPROOTS OF AESCULUS-HIPPOCASTANUM (HIPPOCASTANACEAE)

Aspects of the structure and ultrastructure of the fusiform cambial ce lls of the taproot of Aesculus hippocastanum L. (horse chestnut) are d escribed in relation to the seasonal cycle of cambial activity and dor mancy. Particular attention is directed at cell walls and the microtub ule and microfilament components of the cytoskeleton, using a range of cytochemical and immunolocalization techniques at the optical and ele ctron-microscopical levels. During the dormant phase, cambial cell wal ls are thick and multi-layered, the cells possess a helical array of c ortical microtubules, and microfilament bundles are oriented axially. In the early stages of reactivation, vesicle-like profiles are associa ted with the cell walls, whereas arrangement of the cytoskeletal eleme nts remains unchanged. In the succeeding active phase, the cell walls are thin, and cortical microtubules form a random array, although micr ofilament bundles maintain a near-axial orientation. The observations are discussed in relation to the seasonal cycle of wall structure and cortical microtubule rearrangement within the vascular cambium of hard wood trees. It is suggested that the cell-wall thickening at the onset of cambial dormancy, which is associated with the presence of a helic al cortical microtubule array, should be considered to be secondary wa ll thickening, and that selective lysis of this secondary wall layer d uring cambial reactivation restores the thinner, primary wall found ar ound active cambial cells.