The molecular architecture distinctions between compression, opposite and normal wood of Pinus radiata

Abstract

In gymnosperms compression wood is a specialised type of structural cell wall formed in response to biomechanical stresses. The differences in terms of gross structure, ultrastructure and chemistry are well-known. However, the differences between compression wood, normal wood, and opposite wood regarding the arrangements and interactions of the various polymers and water within their cell walls still needs to be established. The analysis of ¹³C-labelled Pinus radiata by solid-state NMR spectroscopy and other complementary techniques revealed several new aspects of compression and opposite wood molecular architecture. Compared to normal wood, compression wood has a lower water content, its overall nanoporosity is reduced, and the water and matrix polymers have a lower molecular mobility. Galactan, which is a specific marker of compression wood, is broadly distributed within the cell wall, disordered, and not aligned with cellulose, and is found to be in close proximity to xylan. Dehydroabietic acid (a resin acid) is immobilised and close to the H-lignin only in compression wood. Although the overall molecular mobility of normal wood and opposite wood are similar, opposite wood has different arabinose conformations, a large increase in the amount of chain ends, contains significantly more galactan and has additional unassigned mobile components highlighting the different molecular arrangement of cell wall polymers in opposite and normal wood.

Keywords: Pinus radiata (Monterey pine); compression wood (CW); opposite wood (OW); secondary cell wall (SCW); solid state NMR (ssNMR).

Figure 1

A comparison of 1D ¹³C NMR spectrum of normal wood (orange), compression wood (black) and opposite wood (blue). (a) Quantitative (DP 20s), (b) Mobile (DP 2 s) and (c) Immobile (CP). Spectra have been normalized to the C4¹ cellulose peak at 89 ppm (marked with *) and were recorded at a ¹³C Larmor frequency of 150.7 MHz and a MAS frequency of 12 kHz.

Figure 2

Comparison of the C4-C5−C6 region of ¹³C CP-INADEQUATE NMR spectra of compression wood (black) and opposite wood (blue) normalised to the cellulose C4¹ peak at 89 ppm. The spectra were recorded at a ¹³C Larmor frequency 150.7 MHz and a MAS frequency of 12 kHz. The spin-echo duration used was 2.2 ms.

Figure 3

Comparison of the C4-C5−C6 region of ¹³C CP-INADEQUATE NMR spectra of normal wood (orange) and compression wood (black) normalised to the cellulose C4¹ peak at 89 ppm. The normal wood spectrum was recorded at a ¹³C Larmor frequency of 176.0 MHz and a MAS frequency of 12.5 kHz. The compression wood was recorded at a ¹³C Larmor frequency 150.7 MHz and a MAS frequency of 12 kHz. The spin-echo duration used was 2.2 ms.

Figure 4

Comparison of the neutral carbohydrate region of a ¹³C DP-INADEQUATE NMR spectra of never-dried pine (orange) and opposite wood (blue). Novel (Unknown, marked with a question mark) mobile components are present in the opposite wood as well as distinct changes in the arabinoses. Spectra were recorded at a ¹³C Larmor frequency of 150.7 MHz and a MAS frequency of 12 kHz.

Figure 5

Ratio of freezing water for compression, normal, and opposite wood. DSC thermoporosimetry box plot representing the estimated distribution of pore diameter in association with the ratio of freezing water (in percentage). The error bars indicate the 95% confidence interval. The p is the parametric p-value and np is the non-parametric p-value.

Figure 6

Comparison of 1D water edited spectra with a proton filter of 1 ms and a short diffusion time of 1 ms of Compression wood (black), Normal wood (orange), Opposite wood (blue) and a standard 1D CP spectrum of compression wood (black dashed). Spectra were recorded at a ¹³C Larmor frequency of 150.7 MHz and a MAS frequency of 12 kHz.

Figure 7

Left-hand side: A comparison of 400 ms ¹³C CP-PDSD NMR spectra of Opposite wood (blue) and Compression wood (black) normalised to the self-peak of cellulose C1 at 105 ppm. Right-hand side: Slices are taken from the CP PDSD spectrum to highlight the key differences in hemicelluloses between never-dried pine and compression wood: (a) slice at the mannan M1 shift, 101 ppm; (b) slice at the xylan X4 shift, 82.2 ppm; Spectra were recorded at a ¹³C Larmor frequency of 150.7 MHz and a MAS frequency of 12 kHz.

Figure 8

Schematic models which show the differences in the cell wall of (a) normal wood and (b) compression wood as interpreted from the experimental solid-state NMR, XRD and compositional analysis. The models show a transverse section of a cellulose microfibril with a 2-3-4-4-3–2 habit surrounded by the hemicellulose and lignin matrix. (a) Is a four microfibril illustration of our previous never-dried softwood model (Cresswell et al., 2021), where both galactoglucomannan (GGM) and xylan are interacting with the cellulose microfibril surface and there is a range of bound water present in the cell wall. (b) the compression wood model shows the addition of disordered galactan, H-lignin and dehydroabietic acid to the cell wall. The model also illustrates the significant reduction of weakly bound water in the compression wood where it was found that the majority of water present is in a more bound/trapped state.