Responses of wood formation to bending: a matter of dose and sensitivity adjustments

Repeated bending stimulations applied on poplar stem drives wood formation toward egg-shaped cross sections, thicker fiber cell walls and more fibers developing a G-layer; but cells sensitivity accommodates to avoid overresponses.

Trees acclimate to mechanical stimulations (e.g. wind) through thigmomorphogenesis. Recent studies have demonstrated that repetitive unidirectional bending treatments applied to poplar stems result in the production of two distinct types of wood: tensile flexure wood (TFW) on the stretched side and compressive flexure wood (CFW) on the compressed side of the stem. However, the dose-effect responses of wood formation to repeated unidirectional bending treatments have not been established. In this study, we show that the number of bending events plays a crucial role in wood formation. To investigate this, young poplar stems were subjected to two different treatments involving different numbers of transient and unidirectional elastic bends. The radial growth of the stems was monitored throughout the treatments, and wood anatomy was quantitatively analysed and compared to control trees. The elliptic shape of poplar stem cross section, observed in response to the lowest dose, transformed into egg-shaped cross section in response to the highest dose. At the tissue level, the proportion of vessels vs fibers and their sizes were not differentially altered between the two treatments. However, there were notable differences in the proportion of G-fibers and the thickening of secondary cell walls, showing that the different traits of flexure wood have independent mechanosensitive control. Overall, our findings demonstrate that, in addition to their ability to respond to the intensity and direction of local mechanical strains, poplars adjust wood formation based on the number of bending events. These modifications likely enhance stem resistance against breakage when exposed to strong wind gusts.