Ability of selected wood-inhabiting fungi to degrade in vitro sapwood and heartwood of Nothofagus pumilio

Fungi are the main decomposers of lignocellulose in temperate forests, and are classified as either white- or brown-rot, based on the ability to degrade lignin along with cellulose and hemicellulose. In this work, decomposition of Nothofagus pumilio wood by different wood-inhabiting fungal species was investigated through in vitro assays. Sapwood and heartwood blocks were individually exposed to 11 fungal species; mass loss was determined after 75, 135, and 195 days of exposure, comparatively analyzing the fungal ability to colonize and degrade this lignocellulosic substrate corresponding to both parts of the wood. Transverse section slices of the blocks were cut and separately stained with two types of dyes, Congo red and phloroglucinol, that are specifically associated with cellulose and lignin, respectively. Most of the species showed a different performance in sapwood and heartwood. Rhizochaete brunnea, Aurantiporus albidus and Phanerochaete velutina produced the greatest mass losses in sapwood. The latter two and Laetiporus portentosus produced the highest mass losses in heartwood, whereas Rh. brunnea was among the worst decomposers of this substrate. White rotters generally showed a higher ability to degrade the sapwood and brown rotters the heartwood. The fungal species that produced greater mass losses in heartwood than in sapwood grow on heartwood of living trees. Among white-rot fungi, two modes of action were identified: a) localized degradation, with zones of advanced decay in a less deteriorated matrix, and b) homogeneous degradation, with an even decay. Our results showed that many species have different performances in different substrates, reinforcing the importance of analyzing sapwood and heartwood decomposition separately, usually not done in this kind of studies.