Abnormal lignin distribution in wood from severely drought stressed Pinus radiata trees

Radiata pine logs exhibiting concentric shelling were examined for abnormal wood anatomy and cell wall characteristics. The trees from which the logs originated were growing on coastal sand dunes with a shallow impermeable iron pan subsoil, and the abnormal wood properties are assumed to be the result of frequent water stress and possible associated nutritional stress. The wood showed numerous false growth rings alternating with bands of poorly lignified tracheids. Examination of lignin distribution by confocal fluorescence microscopy and transmission electron microscopy revealed abnormal cell wall development associated with a poorly lignified middle lamella and outer secondary cell wall. Affected tracheids showed poor adhesion with development of intercellular checking, particularly on radial cell walls. Some tracheids showed concentric lamellation associated with areas of high and low lignification within the secondary wall. In many cases, the S-3 layer was thicker and more heavily lignified than normal. Tracheids with the greatest reduction in lignification of the secondary wall showed evidence of collapse. The shelling behaviour of the wood was thus explained by poor or negligible adhesion between tracheids due to reduced lignification of middle lamellae. This investigation provides some insight into the effect of growth environment on lignification.