Detection of wood cell wall porosity using small carbohydrate molecules and confocal fluorescence microscopy

A novel approach to nanoscale detection of cell wall porosity using confocal fluorescence microscopy is described. Infiltration of cell walls with a range of nitrophenyl-substituted carbohydrates of different molecular weights was assessed by measuring changes in the intensity of lignin fluorescence, in response to the quenching effect of the 4-nitrophenyl group. The following carbohydrates were used in order of increasing molecular weight; 4-nitrophenyl -D-glucopyrano-side (monosaccharide), 4-nitrophenyl -D-lactopyranoside (disaccharide), 2-chloro-4-nitrophenyl -D-maltotrioside (trisaccharide), and 4-nitrophenyl -D-maltopentaoside (pentasaccharide). This technique was used to compare cell wall porosity in wood which had been dewatered to 40% moisture content using supercritical CO2, where cell walls remain fully hydrated, with kiln dried wood equilibrated to 12% moisture content. Infiltration of cell walls as measured by fluorescence quenching, was found to decrease with increasing molecular weight, with the pentasaccharide being significantly excluded compared to the monosaccharide. Porosity experiments were performed on blocks and sections to assess differences in cell wall accessibility. Dewatered and kiln dried wood infiltrated as blocks showed similar results, but greater infiltration was achieved by using sections, indicating that not all pores were easily accessible by infiltration from the lumen surface. In wood blocks infiltrated with 4-nitrophenyl -D-maltopentaoside, quenching of the secondary wall was quite variable, especially in kiln dried wood, indicating limited connectivity of pores accessible from the lumen surface. Lay Description The subject of this paper is the detection of very small pores in wood. These pores are important for various modification processes that might be involved in making wood resistant to decay, improving the hardness of wood or reducing shrinkage of wood during drying. Pores in the cell walls of wood fibres are too small to see directly with a microscope but they can be better detected by infiltrating with dyes. Because many dye molecules are too large to fit inside the very small wood pores, we have developed an alternative approach where we use small colourless sugar molecules that change the natural fluorescence of the wood when the molecules infiltrate the pores. By using molecules of different sizes, we are able to measure the approximate size and the location of the pores and hence get an idea about what constraints may be involved in modifying wood with chemicals. This method may have applications to study the porosity of other types of materials.