Investigation of tension wood formation and 2,6-dichlorbenzonitrile application in short rotation coppice willow composition and enzymatic saccharification

Abstract

Background: Short rotation coppice willow is a potential lignocellulosic feedstock in the United Kingdom and elsewhere; however, research on optimising willow specifically for bioethanol production has started developing only recently. We have used the feedstock Salix viminalis × Salix schwerinii cultivar 'Olof' in a three-month pot experiment with the aim of modifying cell wall composition and structure within the stem to the benefit of bioethanol production. Trees were treated for 26 or 43 days with tension wood induction and/or with an application of the cellulose synthesis inhibitor 2,6-dichlorobenzonitrile that is specific to secondary cell walls. Reaction wood (tension and opposite wood) was isolated from material that had received the 43-day tension wood induction treatment.

Results: Glucan content, lignin content and enzymatically released glucose were assayed. All measured parameters were altered without loss of total stem biomass yield, indicating that enzymatic saccharification yield can be enhanced by both alterations to cell wall structure and alterations to absolute contents of either glucan or lignin.

Conclusions: Final glucose yields can be improved by the induction of tension wood without a detrimental impact on biomass yield. The increase in glucan accessibility to cell wall degrading enzymes could help contribute to reducing the energy and environmental impacts of the lignocellulosic bioethanol production process.

Figure 1

Diagram of the transverse sections isolated to represent normal, tension and opposite wood fractions. (A) Diagram of a transverse section of one of the three trees induced over 43 days by inclining. The two areas indicated by dashed lines represent the isolated tension wood and opposite wood fractions. (B) Diagram of a transverse section of one of the three control trees grown with the 43-day-treated trees. The single area indicated by the dashed line represents isolated fraction equivalent to reaction wood.

Figure 2

Transverse sections of 26-day treatment groups. Midpoint 20- μm transverse sections from the 26-day treatment groups stained in 1% Chlorazol Black E in methoxyethanol and 1% Safranin O (aqueous) of (A) control tree, (B) 2,6-dichlorobenzonitrile (DCB)-treated tree, (C) tension wood-induced tree and (D) tension wood-induced, DCB-treated tree. Scale bar = 1 mm. (E) Tension wood-induced, DCB-treated tree. Scale bar = 100 μm. Tension wood- and DCB-affected regions are highlighted by vertical double arrows.

Figure 3

Enzymatic saccharification. Enzymatic saccharification of control, tension wood-induced, 2,6-dichlorobenzonitrile (DCB)-treated, and tension wood-induced, DCB-treated trees in the 26-day treatments group (26d, n = 6). Debarked control trees and isolated tension and opposite wood are also included for comparison in the 43-day treatment group (43d, n = 3). (A) g (grams) released glucose (g^-1 dry matter (DM)) and B) g released glucose (g^-1 glucan) Error bars = 1 standard error of the mean. Results of Tukey's Honestly Significant Difference (HSD) post hoc test (α = 0.05) are represented by a, b, c, d, e and f.

Figure 4

Compositional analysis. Composition of control, tension wood-induced, 2,6-dichlorobenzonitrile (DCB)-treated, and tension wood-induced, DCB-treated trees in the 26-day treatment group (n = 6). Debarked control trees and isolated tension and opposite wood are also included for comparison in the 43-day treatment group (n = 3). (A) Glucan content expressed as the percentage of dry matter (% DM). (B) Total lignin content expressed as % DM. Error bars = 1 standard error of the mean. Results of Tukey's Honestly Significant Difference (HSD) post hoc test (α = 0.05) are represented by a, b, c, d, e and f.