Dimensional Changes of Tracheids during Drying of Radiata Pine (Pinus radiata D. Don) Compression Woods: A Study Using Variable-Pressure Scanning Electron Microscopy (VP-SEM).

Miao Zhang,Philip Harris,Ramesh Chavan,Bronwen Smith,Brian Mcardle,Bryony James

doi:10.3390/plants7010014

Abstract

Variable-pressure scanning electron microscopy was used to investigate the dimensional changes in longitudinal, tangential and radial directions, on wetting and drying, of tracheids of opposite wood (OW) and three grades of compression woods (CWs), including severe CW (SCW) and two grades of mild compression wood (MCW) (MCW1 and MCW2) in corewood of radiata pine (Pinus radiata) saplings. The CW was formed on the underside and OW on the upper side of slightly tilted stems. In the longitudinal direction, the shrinkage of SCW tracheids was ~300% greater than that of OW tracheids, with the shrinkage of the MCW1 and MCW2 tracheids being intermediate. Longitudinal swelling was also investigated and hysteresis was demonstrated for the tracheids of all corewood types, with the extent of hysteresis increasing with CW severity. A statistical association was found between longitudinal shrinkage and the content of lignin and galactosyl residues in the cell-wall matrix. The galactosyl residues are present mostly as (1→4)-β-galactans, which are known to have a high capacity for binding water and swell on hydration. The small proportions of (1→3)-β-glucans in the CWs have similar properties. These polysaccharides may play a functional role in the longitudinal shrinking and swelling of CW tracheids. Tangential shrinkage of tracheids was greater than radial shrinkage but both were greatest for OW and least for SCW, with the MCW1 and MCW2 being intermediate.

Highlights

In coniferous gymnosperms, compression wood (CW) is a reaction wood formed when growing stems are tilted from the vertical as the result of various factors such as the weight of snow or the slope of the land
Non-linear relationships were observed between relative humidity and percentage dimensional change and for all the corewood types, with severe CW (SCW) showing the greatest change, opposite wood (OW) the least and MCW1 and MCW2 intermediate
Our variable-pressure scanning electron microscopy (VP-SEM) study of radiata pine corewood types showed that the tracheids in discs of mild compression wood (MCW)

Summary

Introduction

In coniferous gymnosperms (softwoods), compression wood (CW) is a reaction wood formed when growing stems are tilted from the vertical as the result of various factors such as the weight of snow or the slope of the land. CW forms on the lower side of tilted stems to correct the growth to the vertical [1]. The wood formed on the opposite side of the stem is referred to as opposite wood (OW) and is very similar in structure and composition to normal wood (NW). Plants 2018, 7, 14 and rewetted, it shrinks and swells and this dimensional instability is a major obstacle in the use of wood, especially for structural purposes [2]. CW shows high longitudinal shrinkage and swelling [1]. Because of the differences in longitudinal shrinkage and swelling between CW and OW or NW, where these wood types occur adjacent to one another in timber, severe warping can occur with a loss of value [2]. CW shows differences from OW and NW in tangential and radial shrinkage but the differences are much less pronounced [1]

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