Subcellular level impact of extractives on brown rot decay of Norway spruce elucidated by confocal Raman microscopy and multivariate data analysis

The influence of heartwood extractives on the chemical changes occurring during the first two weeks of brown rot decay of Norway spruce (Picea abies) latewood tracheids was investigated using confocal Raman imaging in combination with data decomposition by multivariate curve resolution-alternating least squares (MCR-ALS), followed by clustering by K-means cluster analysis (KMCA). The (early) putatively non-enzymatic, oxidative phase was spatially separated from the early enzymatic phase of Rhodonia placenta decay by a directional growth setup. In accordance with the known low decay resistance of Norway spruce, extractives only showed limited ability to prevent oxidative changes. Despite the high co-localization of lignin and extractives, the first signs of decay were seen in the lignin-rich cell wall compartments and seemed to affect lignin itself. When present, the extractives were able to delay the disruption of the aromatic backbone of the lignin polymer during the initial non-enzymatic phase of decay but not of the aliphatic moieties. This indicates that the extractives in Norway spruce appear to reduce the rate of fungal progression mainly by delaying the degradation of lignin in the lignin-rich cell wall layers.