Electron microscopical observations and chemical analyses supporting Mn uptake in white rot degradedAlstoniaand pine wood stakes exposed in acid coniferous soil

Abstract

Brownish-black spots and flecks several millimetres wide were frequently recognized on the surface and within nonpreservative-treated hardwood (Alstonia scholaris (R.Br.)) and softwood (Pinus sylvestris L.) test stakes placed in an acid forest soil. Energy-dispersive X-ray microanalysis in conjunction with transmission electron microscopy (TEM) and gross chemical analysis of wood using inductively coupled plasma spectrometry showed the flecks to be composed primarily of Mn, which was selectively removed from the surrounding soil. A similar uptake of Mn into wood stakes placed in presterilized acid soil was not noted, indicating the process resulted from biotic activity. Detailed TEM observations showed intrusion of Mn into the wood cell lumina and into areas of erosion, cavity formation, and decayed middle lamella in Alstonia and pine wood cells attacked by an unknown white rot decay fungus. Distinct zones of apparent delignification were also noted progressing across secondary cell walls and middle lamella regions of attacked cells, although it was unclear if the effect was caused by nonenzymatic attack by Mn, enzymatic attack by the fungus, or a combination of both. Mn is thought to play a major regulating role in both lignin depolymerization and mineralization in the presence of organic acids during white rot decay. Present observations also suggest that uptake of Mn into wood stakes during microbial degradation results from biotic activity and soil type and pH are of major significance.Key words: Mn, test stakes, electron microscopy, white rot decay, inductively coupled plasma spectrometry.