Cell wall ultrastructure of the lightest wood on earth

The cell wall structure of the ultralight wood from a swamp forest tree, Alstonia spatulata Blume (Apocynaceae), was studied using transmission electron microscopy. The wood sample was derived from rootwood collected in Java as a part of the Kanehira collection. The ground tissue is composed of “modified fibres”, which almost lack intrusive tip growth. Despite the extremely thin fibre walls, they show a three-layered structure consisting of a S1, S2 and S3 layer, which is typical of fibre walls. However, the S2 layer has an unusually high microfibril angle of 45–60 degrees. The large, distinctly vestured inter-fibre pits on radial walls have significantly reduced pit borders, which scarcely cover the pit chambers, and mostly lack pit membranes. Vessel walls have a four-layered structure that is one of the common types found in hardwood species. Xylem parenchyma cell walls (both ray and axial) are very thin, resembling fibre walls in their three-layered ultrastructure, with an amorphous layer as the innermost layer. The extremely low frequency of relatively narrow vessels in the stem base and roots likely cannot meet the demands of hydraulic conduction. However, this limitation is likely compensated by the “modified fibres”, which overwhelmingly dominate the ground tissue and have thin walls with wide-apertured pits that lack pit membranes. “Modified fibres”, with their extremely thin and flat-helical cell wall structure, are not necessarily weak in resisting axial compression. Nevertheless, our current knowledge of the cell wall structure does not fully resolve the physiological and biomechanical conundrum of how small- to medium-sized trees can be supported by such a mechanically weak base.

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