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The work presented shows that wounding of the xylem may induce a modified wall architecture and lignin distribution in tissues differentiating at the time of wounding. An increasing lignin content and distinctively thickened walls can contribute to improved resistance as part of the compartmentalization process.

Information about fine structure following wounding in differentiating xylem tissue is still scarce. This study provides information on cell wall modifications with special emphasis on lignin distribution in xylem fibres of poplar differentiating at the time of wounding. Samples were collected from wounded Populus spp. trees after response periods of up to twenty-three months and processed for microscopic analyses. General studies on the wall structure of wound-adjacent xylem fibres were carried out with light and transmission electron microscopy, whereas lignin distribution patterns of these cells were examined by UV-microspectrophotometry.Xylem fibres close to the wound and within a transition zone between differentiated xylem laid down prior to and tissue laid down after wounding developed a distinctively thicker secondary wall than normal fibres. These modified walls also showed a slightly higher lignin content, than normal and a heterogeneous lignin distribution in the middle lamella and the secondary wall.Wounding in poplar induces a modified wall structure and lignin topochemistry in xylem fibres differentiating at the time of wounding. It is assumed that this wound response is part of the compartmentalization process and therefore contributes to an increased resistance.

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Cell wall modifications in vessels and fibres of wound wood of Populus tremula L.×P. tremuloides Michx. formed after mechanical wounding have been examined by light microscopy, transmission electron microscopy, and UV microspectrophotometry (in scanning and point measurement mode), mainly focusing on the lignin distribution. With this goal, wound xylem within lateral wound callusis was collected after response periods of up to 23 months. Vessels and fibres in wound xylem deviated from their usual axial orientation. Vessels within the wound xylem were smaller in diameter and shorter in length. Xylem fibres were also shorter and developed thicker walls, especially in tissue adjacent to the wound. Cell walls and cell corners of these fibres showed on average a higher lignin content and a modified lignin composition. These wall changes probably enhance disease resistance of the wound tissue. With increasing distance from the wound edge, the modifications diminished and finally disappeared.