Distribution of acidic and esterified polygalacturonans in sapwood of spruce, birch and aspen

The distribution and degree of esterification of polygalacturonans have been studied in sapwood of spruce, birch and aspen by immunolocalisation using monoclonal antibodies. The antibodies, called JIM7 and JIM5, can specifically differentiate between high methyl esterified polygalacturonan (JIM7) and low methyl esterified polygalacturonan (JIM5). Polygalacturonan is a major constituent in pectin. thereby, localisation of polygalacturonans revealed the distribution of pectic substances in the wood samples investigated in this study. Pectin was localised to the compound middle lamella between fibre walls and in cell junctions. Pectin was also present in ray cell walls and in pit membranes. Generally, pectin was mainly present in highly methyl esterified form in sapwood, although some acidic pectin was also found. In spruce the methylated form of pectin was more abundant than acidic pectin. However, the distributions of methylated and acidic pectins were similar. In birch there was a significantly higher amount of methylated, compared to acidic, pectin present. Also, aspen sapwood contained more methylated than acidic pectin. A notable feature of aspen was the different distribution pattern of pectin, where both methylated and acidic pectins were distinctively concentrated to the middle lamella cell corners. The implications of the presence of pectin in wood cell walls upon the utilisation of wood in technical processes are discussed.