Flexure wood is not a single tool for coping with wind strain

Investigating flexure wood, a new paper finds that the cells in a tree react to strain from wind differently depending on whether they are being stretched or compressed.

Outdoors is usually the best place for a tree, but it’s not without its stresses and strains. Wind is almost constant to some degree, pushing this way or that. It means that the tree has to modify is growth to respond. This is called thigmomorphogenesis. To break it down, thigmo refers to touch, morpho to the shape and genesis to creation. This describes reactions like the shoot growing shorter but wider to fight strains, as well as increased anchoring of the roots.

Bent Tree
A tree on a calm day on Stanton Moor.

A paper in Annals of Botany looks at a tree’s reaction to wind stresses, not through this response, but through another method that a tree tackles strain: flexure wood. This is the wood that trees create to counteract forces pushing on them. This has been studied, mainly in gymnosperms, and researcher have found that you get ovalization as well as flexure wood. However, there hasn’t been the same research for angiosperm trees.

Roignant and colleagues set out to investigate this. They started by thinking about the strain the tree was under. When the wind bends a tree one side is compressed, but the other side is stretched, so it’s under a tensile strain. These are two very different situations, so they thought to look for different responses.

They way they tested responses was by bending poplar stems. For eight weeks, they carefully added set strains on a stem for measured lengths of time – always in the same direction. This control over direction meant they knew exactly which side of the stem was compressed and which was stretched.

What the team found was that when you bend an angiosperm, there’s a reaction on all sides of the plant, but it’s strongest in the direction that you flex the plant. This happens on both sides, and that’s how the stem turns from a round cross-section to an oval. They also found that vessel frequency and size changed, frequency on both sides, but size only on one.

Looking for the mechanism, the authors found that cell wall-related genes were over-expressed in the compressed wood by a factor of 3 to 4 and slightly less in the tension wood. The physiology of the wood also differed. Vessel frequency decreased by 19% in bent stems compared with control trees. On the tension side of the stem, vessel diameter also fell by over 8%.

Roignant et al. say that these differences mean that we can differentiate between types of flexure wood. Instead, we have tensile flexure wood (TFW) the wood created by stretching and compressive flexure wood (CFW). It means that a plant can react differently to positive and negative strains, so not all flexure wood is the same.

Reference List

Chehab, E. W., Eich, E., & Braam, J. (2008). Thigmomorphogenesis: a complex plant response to mechano-stimulation. Journal of Experimental Botany, 60(1), 43–56. https://doi.org/10.1093/jxb/ern315

Roignant, J., Badel, É., Leblanc-Fournier, N., Brunel-Michac, N., Ruelle, J., Moulia, B., & Decourteix, M. (2018). Feeling stretched or compressed? The multiple mechanosensitive responses of wood formation to bending. Annals of Botany, 121(6), 1151–1161. https://doi.org/10.1093/aob/mcx211