Lignification and cell wall thickening of ray parenchyma cells in Scots pine sapwood

Scots pine exhibits variations in ray anatomy, which are poorly understood. Some ray parenchyma cells develop thick and lignified cell walls before heartwood formation. We hypothesized that some stands and trees show high numbers of lignified and thick-walled parenchyma cells early in the sapwood. Therefore, a microscopic analysis of Scots pine sapwood from four different stands in Northern Europe was performed on Safranin — Astra blue-stained tangential micro sections from outer and inner sapwood areas. Significant differences in lignification and cell wall thickening of ray parenchyma cells were observed in the outer sapwood between all of the stands for the trees analyzed. On a single tree level, the relative lignification and cell wall thickening of ray parenchyma cells ranged from 4.3% to 74.3% in the outer sapwood. In the inner sapwood, lignification and cell wall thickening of ray parenchyma cells were more frequent. In some trees, however, the difference in lignification and cell wall thickening between inner and outer sapwood was small since early lignification, and cell wall thickening was already more common in the outer sapwood. Ray composition and number of rays per area were not significantly different within the studied material. However, only one Scottish tree had a significantly higher number of ray parenchyma cells per ray. The differences discovered in lignification and cell wall thickening in ray parenchyma cells early in the sapwood of Scots pine are relevant for wood utilization in general and impregnation treatments with protection agents in particular.