Abstract
BackgroundSalix (willow) species represent an important source of bioenergy and offer great potential for producing biofuels. Salix spp. like many hardwoods, produce tension wood (TW) characterized by special fibres (G-fibres) that produce a cellulose-rich lignin-free gelatinous (G) layer on the inner fibre cell wall. Presence of increased amounts of TW and G-fibres represents an increased source of cellulose. In the present study, the presence of TW in whole stems of different Salix varieties was characterized (i.e., physical measurements, histochemistry, image analysis, and microscopy) as a possible marker for the availability of freely available cellulose and potential for releasing d-glucose. Stem cross sections from different Salix varieties (Tora, Björn) were characterized for TW, and subjected to cellulase hydrolysis with the free d-glucose produced determined using a glucose oxidase/peroxidase (GOPOD) assay. Effect of cellulase on the cross sections and progressive hydrolysis of the G-layer was followed using light microscopy after staining and scanning electron microscopy (SEM).ResultsTension wood fibres with G-layers were developed multilaterally in all stems studied. Salix TW from varieties Tora and Björn showed fibre G-layers were non-lignified with variable thickness. Results showed: (i) Differences in total % TW at different stem heights; (ii) that using a 3-day incubation period at 50 °C, the G-layers could be hydrolyzed with no apparent ultrastructural effects on lignified secondary cell wall layers and middle lamellae of other cell elements; and (iii) that by correlating the amount of d-glucose produced from cross sections at different stem heights together with total % TW and density, an estimate of the total free d-glucose in stems can be derived and compared between varieties. These values were used together with a literature value (45%) for estimating the contribution played by G-layer cellulose to the total cellulose content.ConclusionsThe stem section-enzyme method developed provides a viable approach to compare different Salix varieties ability to produce TW and thus freely available d-glucose for fermentation and biofuel production. The use of Salix stem cross sections rather than comminuted biomass allows direct correlation between tissue- and cell types with d-glucose release. Results allowed correlation between % TW in cross sections and entire Salix stems with d-glucose production from digested G-layers. Results further emphasize the importance of TW and G-fibre cellulose as an important marker for enhanced d-glucose release in Salix varieties.
Highlights
Salix species represent an important source of bioenergy and offer great potential for producing biofuels
An important parameter for selection of any plant/ wood species for biofuel production is based on the lignocellulose material recalcitrance, which is related to the chemistry and relation between the major biomass polymers cellulose, hemicelluloses and lignin
We developed a method to assessed entire stem cross sections from different Salix varieties and applied a commercial cellulase to determine accessible cellulose as an indirect method to quantify the amount of tension wood (TW) tissue and readily accessible d-glucose
Summary
Salix (willow) species represent an important source of bioenergy and offer great potential for producing biofuels. An important parameter for selection of any plant/ wood species for biofuel production is based on the lignocellulose material recalcitrance, which is related to the chemistry and relation between the major biomass polymers cellulose, hemicelluloses and lignin. Cellulose in the biomass is usually converted to fermentable sugars through thermal- or chemical pretreatments. This is followed by enzymatic hydrolysis with enzymes (i.e., cellulases), the glucose simultaneously converted by yeast or conducted in a separate process [1]. For bioethanol production, the most important and limiting factors will be the total cellulose content and its accessibility in the lignocellulose biomass structure for enzymatic hydrolysis
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