Lignification of spruce tracheid secondary cell walls related to longitudinal hardness and modulus of elasticity using nano-indentation

The lignin content and the mechanical properties of lignifying and fully lignified spruce tracheid secondary cell walls were determined using UV microscopy and nano-indentation, respectively. The average lignin content of developing tracheids was 0.10 g.g(-1), as compared with 0.21 g.g(--1) in mature tracheids. The modulus of elasticity of developing cells was on average 22% lower than the one measured in mature, fully lignified cells. For the longitudinal hardness, a larger difference of 26% was observed. As lignifying cells in the cambial zone are undergoing cell wall development, spaces in the cellulose-hemicellulose structure are filled with lignin and the density of the cell wall is believed to increase. It is therefore suggested that the observed difference in modulus of elasticity between developing and fully lignified cell walls is due to the filling of spaces with lignin and an increase of the packing density of the cell wall during lignification. Although remarkably less stiff than the composite polysaccharide structure in the secondary cell wall, lignin may be considered equally hard. Therefore, the observed increase in lignin content may contribute directly to the measured increase of hardness.