Ultrastructural Investigation of Tension Wood Fibre in Fraxinus mandshurica Rupr. var. japonica Maxim.

Abstract

The ultrastructure of the fibre wall in Fraxinus mandshurica Rupr. var. japonica Maxim. was investigated by electron microscopy. The trees had been inclined artificially at an angle of 30° to the vertical at the beginning of the initiation of cambial growth in early spring. The secondary walls of tension wood fibres were of the outer (S 1) layer and gelatinous (G) layer type. The microfibrils in the gelatinous (G) layer were oriented as a steep Z-helix relative to the fibre axis with a deviation that ranged from 0° to 25° but was mainly between 5° and 10°. The cross-sectional surface of tension wood fibres revealed the relatively strong attachment of the G-layer to the S 1 layer. The G-layer stained weakly with potassium permanganate. The S 1 layer of tension wood fibres stained less strongly than that of the normal and opposite wood fibres. These results indicate that the tension wood in F. mandshurica var. japonica is not typical and is somewhat anomalous. The secondary walls of normal and opposite wood fibres were composed of two layers, S 1 and S 2, and lacked an S 3 layer. Microfibrils in the S 3 layer of juvenile stems were extremely variable in orientation and were sparsely distributed without forming a layer. By contrast, a very thin S 3 layer was present in the wood fibres of mature stems. The variations in the formation of the S 3 layer in the fibre walls were probably due to the differences in the cambial age of the stems of F. mandshurica Rupr. var. japonica.