Understanding the processes allowing trees to orientate their stems and branches requires an understanding of the mechanical properties of individual cells. As the cells are formed, maturation stresses are created that can lead to the reorientation of the tree. But measuring the properties within the different wood cells produced in normal wood, tension wood or compression wood requires measurements at very fine spatial resolution and the wood cells must remain in-situ so that the cell mechanical characteristics are preserved. In the article of Arnould et al (2022), measurements of the mechanical characteristics of poplar tension wood were measured in comparison to normal wood at different distances from the cambium and therefore different levels of maturation. The work required incredible care to embed the wood in resin, to cut the wood with extremely sharp microtone blades in order to minimize artefacts in the measurements, and then ultra-careful atomic force microscope (AFM) measurements across cell walls from the edge of the lumen to the middle lamella at extremely high spatial resolution. The result is a detailed picture of the kinetic development and maturation of tension wood cells in a tree. The measurements showed that the G-layer reaches close to its final stiffness long before its final thickness, and this is different from the maturation kinetics of other cell wall layers where thickening and stiffening are generally synchronous. Finally, although the G-layer in poplar tension wood fibres and in flax phloem fibres are in many ways very similar there are clear differences in the kinetics of their development and maturation. The detailed information presented in this paper can therefore help to clarify the different hypothetical mechanisms proposed to explain excess stress generation in the tension wood of trees and help move us towards a full understanding of how the “muscles” of trees work.

References

Arnould O, Capron M, Ramonda M, Laurans F, Alméras T, Pilate G, Clair B (2022) Mechanical characterisation of the developing cell wall layers of tension wood fibres by Atomic Force Microscopy. bioRxiv, 2021.09.23.461481, ver. 4 peer-reviewed and recommended by Peer Community in Forest and Wood Science. https://doi.org/10.1101/2021.09.23.461481