Resistance of parenchyma cells in wood to degradation by brown rot fungi

Wood degradation by two basidiomycetes, Fomitopsis pinicola and Laetiporus sulphureus was studied in one conifer and four broadleaved trees: Picea abies (Norway spruce), Acer pseudoplatanus (sycamore), Betula pendula (birch), Quercus robur (common oak) and Robinia pseudoacacia (robinia). Observations of birefringence under polarized light showed that in all hosts both brown rot fungi affected cells of the early wood before those of the late wood. Degradation of cellulose, as shown by the loss of birefringence, was apparent after 6 weeks in the cell wall of fibres and fibre tracheids, but even after 12 weeks, axial parenchyma showed no signs of degradation. The results indicate that both brown rot fungi cause higher weight losses in hosts (P. abies and B. pendula) with a small amount of parenchyma cells, whereas the lowest weight losses are associated with wood containing a high amount of parenchyma cells (Q. robur and R. pseudoacacia). Resistance of parenchyma cells to degradation by brown rot fungi appears to be related to the cell wall morphology of parenchyma cells and may also reflect a low co-evolutionary adaptation of brown rot fungi to the xylem of broadleaved trees.