Immunocytochemical studies of axial resin canals. II. Localization of non-cellulosic polysaccharides in epithelium and subsidiary cells of Scots pine

Axial resin canals in wood are distinguished into two types based on the morphology of epithelial cells; resin canals with narrow canals and thick-walled epithelial cells (Type I), and resin canals with wide canals and thin-walled epithelial cells (Type II). Following studies on Norway spruce (Type I), the distribution of non-cellulosic polysaccharides in axial resin canals of Scots pine (Type II) is reported here using cytochemical and immunocytochemical methods. The distribution of (1→4)-β-galactan (LM5), (1→5)-α-arabinan (LM6), homogalacturonan (LM19, LM20), xyloglucan (LM15), xylan (LM10, LM11) and mannan (LM21, LM22) epitopes were examined. Axial resin canal complexes in the xylem were composed of canal, epithelium and subsidiary cells (parenchyma and strand tracheids). Strand tracheids were absent in axial resin canals in the phloem. Strand tracheids showed a completely different ultrastructure and chemistry from normal mature tracheids and other types of axial resin canal cells. Immunolocalization of non-cellulosic polysaccharides in axial resin canals showed an overall similar cell wall composition in epithelial cells and subsidiary parenchyma between the xylem and phloem. All types of axial resin canal cells in both xylem and phloem contained homogalacturonan (HG), rhamnogalacturonan-I (RG-I) and xyloglucan with a high variation in amount and chemical structure depending on cell wall region and between cell types. In particular, epithelial cell walls facing the canal showed significant differences in HG distribution from other epithelial cell wall regions. No xylan and mannan epitopes were detected in any of axial resin canal cells. Together, our results suggest that the chemistry of axial resin canal cells in Scots pine may be highly compartmentalized depending on functional differences between both cell types and cell wall regions.