Solar radiation affected the wood elements of both axial system and radial systems. Weathering of wood from solar radiation is well documented and is caused primarily by the action of UV radiation on the lignin component of wood cell walls. The middle lamella, consisting largely of lignin, and which holds adjoining wood cells together, loses its integrity due to the breakdown of lignin from the effect of UV radiation, enabling the cells to separate from one another. UV radiation may also break down lignin present in other parts of wood cell walls. However, the separation of cells occurs before the cell walls break down completely, implying that breakdown of middle lamella lignin is a more significant factor in coating failure.
Cells can also separate due to failure in the middle lamella in the tangential direction.
Combined radial and tangential failures in the middle lamella cause the cells to separate individually. Failure of the middle lamella largely in the radial direction results in the separation of rows of cells. Although ray parenchyma in the sapwood of radiata are not lignified, UV radiation can also cause separation in rays. Ray parenchyma can separate from neighbouring tracheids due to loss of lignin in the latter cell type, and in extreme cases the entire ray parenchyma cells can collapse due to thinning of unlignified walls. Loss in the integrity of ray parenchyma is responsible for much of the checking found in wood exposed outdoors.