Application of micro-PIXE, MRI and light microscopy for research in wood science and dendroecology

Beech (Fagus sylvatica L.) branches were topped and after five months the wound response was analyzed by PIXE, 3D-MRI and light microscopy. From freshly cut and deeply frozen sample 30 μm thick longitudinal-radial tissue sections were prepared for anatomical investigations and micro-PIXE analysis. Light microscopy revealed the structural response to wounding, i.e. occurrence of the reaction zone between the exposed and dehydrated dead tissue and healthy sound wood. The reaction zone was characterized by tylosis in vessels and accumulation of colored deposits in parenchyma cells, fibres and vessels. 3D MRI of a parallel sample showed that the moisture content in the reaction zone was three times higher than in normal healthy wood. Micro-PIXE mapping at margins of compromised wood in beech revealed an increased concentration of potassium in the reaction zone. The increase in the calcium concentration was associated with the dehydrated tissue adjacent to reaction zones.In addition, micro-PIXE was used to determine the elemental distribution in annual tree rings. This may be relevant for retrospective assessment of environmental pollution in wood by measuring yearly increments as a biomonitoring tool. The analysis of European larch (Larix decidua Mill.) wood revealed a high similarity between optical characteristics (i.e. late versus earlywood) and elemental (e.g. Cl, K, Ca, Mn, Zn) distribution.