ALTERATIONS OF PARENCHYMA PITS DUE TO WOUND REACTIONS IN BIRCH XYLEM (BETULA-PENDULA ROTH)

Branches of Betula pendula Roth were wounded in spring and summer. Samples near the wound were dissected after 1 and 3 weeks, prepared, and analysed by Transmission Electron Microscopy. Parenchyma cells at the wound begin to synthesize fibrillar material, which is extruded into adjacent vessels and fibres with subsequent occlusion of the lumina. Before secretion starts, the pit membranes as well as the Protective Layer of the half-bordered pits between these cells show a continuous toss of electron density, which may be a sign of increased permeability; with secretion those features appear intensified. At late stages the pit membranes consist of loosely packed fibrils representing the cellulosic network. With increasing structural alterations of the pit membranes a simultaneous decrease in intensity of the PATAg-reaction occurs, mainly indicating a loss of hemicelluloses. The reduced amount of incrusting material and finally its complete loss renders an efficient secretion of fibrillar material from parenchyma cells through altered pit membranes into adjacent vessels and fibres.