Abstract
BackgroundCompression wood (CW) forms on the underside of tilted stems of coniferous gymnosperms and opposite wood (OW) on the upperside. The tracheid walls of these wood types differ structurally and chemically. Although much is known about the most severe form of CW, severe CW (SCW), mild CWs (MCWs), also occur, but less is known about them. In this study, tracheid wall structures and compositions of two grades of MCWs (1 and 2) and SCW were investigated and compared with OW in slightly tilted radiata pine (Pinus radiata) stems.ResultsThe four wood types were identified by the distribution of lignin in their tracheid walls. Only the tracheid walls of OW and MCW1 had a S3 layer and this was thin in MCW1. The tracheid walls of only SCW had a S2 layer with helical cavities in the inner region (S2i). Using immunomicroscopy, (1 → 4)-β-D-galactans and (1 → 3)-β-D-glucans were detected in the tracheid walls of all CWs, but in only trace amounts in OW. The (1 → 4)-β-D-galactans were located in the outer region of the S2 layer, whereas the (1 → 3)-β-D-glucans were in the inner S2i region. The areas and intensities of labelling increased with CW severity. The antibody for (1 → 4)-β-D-galactans was also used to identify the locations and relative amounts of these galactans in whole stem cross sections based on the formation of an insoluble dye. Areas containing the four wood types were clearly differentiated depending on colour intensity. The neutral monosaccharide compositions of the non-cellulosic polysaccharides of these wood types were determined on small, well defined discs, and showed the proportion of galactose was higher for CWs and increased with severity.ConclusionThe presence of an S3 wall layer is a marker for very MCW and the presence of helical cavities in the S2 wall layer for SCW. The occurrence and proportions of (1 → 4)-β-D-galactans and (1 → 3)-β-D-glucans can be used as markers for CW and its severity. The proportions of galactose were consistent with the labelling results for (1 → 4)-β-D-galactans.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-016-0884-3) contains supplementary material, which is available to authorized users.
Highlights
Compression wood (CW) forms on the underside of tilted stems of coniferous gymnosperms and opposite wood (OW) on the upperside
Two grades of Mild compression wood (MCW), Mild compression wood 1 (MCW1) and Mild compression wood 2 (MCW2), as well as severe CW (SCW) and OW were identified in radiata pine and characterized
A S3 tracheid wall layer was present in only OW and the mildest MCW, MCW1, and helical cavities were present only in the inner region of the S2 tracheid wall layer in SCW. (1 → 4)-β-Galactans were a characteristic feature of the outer S2 layer (S2L) tracheid wall layer of all grades of CW and not just SCW, with the proportions increasing with CW severity
Summary
Compression wood (CW) forms on the underside of tilted stems of coniferous gymnosperms and opposite wood (OW) on the upperside. The tracheid walls of these wood types differ structurally and chemically. When the growth of stems of such woody plants is displaced from the vertical, for example by wind or snow, a special type of secondary xylem is formed known as reaction wood, which restores normal, vertical growth [1, 2]. In coniferous gymnosperms (softwoods), this reaction wood is formed on the underside of tilted stems and is known as compression wood (CW) [2]. The wood formed geometrically opposite to CW, is known as opposite wood (OW) and the cell walls of this wood type are structurally and chemically similar to those of NW [4]. CW shrinks longitudinally more than NW, and when CW occurs with NW, the differential shrinkage causes warping and other distortions [7]
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