Abstract
Trees can control their shape and resist gravity thanks to their ability to produce wood under tensile stress at their periphery. This prestress is known to be produced during the maturation of wood fibres but its generation mechanism remains unclear. This study focuses on the wood formation process at two levels: i) at the tissue level, the process and timing of tension wood, opposite wood and normal wood formation were investigated on field grown poplar trees and ii) at the cell wall level, the formation of the secondary wall in tension wood was studied in artificially tilted poplar saplings. Results showed that the number of cambial cells at early growing season, and thus the total number of cells produced in the end, on the tension wood side of bent trees increased compared to opposite and normal wood. The total number of opposite wood cells produced obviously decreased as a result of a lower cambial activity on this side. Consequently, one can observe pith eccentricity in the bent trees. The lignification phase starts latter in opposite wood than in tension and normal wood, however no obvious differences were observed between tension and normal wood. The so-called G-layer (gelatinous layer) formed soon after the start of the lignification in tension wood. Although the total number of cells produced on tension wood side was more important than the averaged one produced in upright trees, the total number of cells produced in the whole growth ring of bent trees was similar to the one produced in upright trees. This was the result of a huge reduction in the number of cells produced on the opposite wood side of bent trees. Tilting also had obvious effect on the wall thickening of young poplar. The thickness of the secondary wall layer and G-layer were measured, from cambium to mature wood, in several trees sampled at different times after tilting. Measurements on wood fibres produced before tilting show the usual progressive increase of secondary wall thickness during the growing season. After the tilting date, the secondary layer thickness decreased markedly from normal wood to tension wood while the total thickness increased, compared to normal wood, with the development of a thick G-layer. However, even after the G-layer formation, the secondary layer thickness continues to increase during the growing season. G-layer thickening was observed to be faster than secondary layer thickening. The development of the unlignified GL is proposed to be a low cost but efficient strategy for a fast generation of high tensile stress in hardwood trees.