11-31: Enzymatic degradation in relation to plant tissues and cell wall components in wheat straw (Triticum aestivum L.)

Wheat straw used for bioethanol varies in digestibility according to the amounts and types of polysaccharides, lignin and minerals constituting the cell walls. The abundance of these components varies between cell types and tissues and consequently also between plant organs, e.g. leaves and stems. This study investigated enzymatic digestibility of leaves and stems, the two main plant organs in wheat straw, and related the differences to cell wall structures and tissue types. Wheat leaves and stems were hydrothermally pretreated separately on lab-scale and subsequently dried or dewaxed before enzymatic hydrolysis. For comparison, industrially pretreated wheat straw was also included in the experimental setup.

Atomic Force Microscopy (AFM) was used to morphologically characterise the sample surfaces on nanoscale and Scanning Electron Microscopy (SEM) was applied to image structural changes on tissue level. Using labelled cellulases, Confocal Laser Scanning Microscopy was used to image enzyme affinity to particular tissues. Chemical surface analysis was performed using Attenuated Total Reflectance-Fourier Transform Infrared spectroscopy (ATR-FTIR) together with composition analysis.

Our investigations support parenchyma tissues to be the most digestible tissue type. As wheat leaves mainly consist of parenchyma tissue, leaves are more digestible than stems. The results also suggest that high initial conversion rates observed in other studies of unfragmented plant material could be related to available tissues, i.e. high initial amounts of parenchyma tissues available for enzymatic digestion. Fractionation of plants into leaves and stems or promotion of plant cultivars containing more parenchyma tissues could provide an opportunity for more cost efficient biomass conversion.