Topochemical studies on the wall of beech bark sclereids by enzymatic and acidic degradation.

Sclereids isolated from the bark of beech ( Fagus sylvatica L.) were delignified and treated with 1.3% sulfuric acid or with purified enzymes, viz., avicelase, carboxymethylcellulase, xylanase as well as combinations of xylanase and avicelase. Monitoring of the degradation was performed by quantitative liquid chromatography. Sulfuric acid dissolves about 30% sugars, especially hemicelluloses after 12 hours treatment. The avicelase (cellulase) and carboxymethylcellulase treatment degraded cellulose only to a very small extent. The xylanase degraded xylan selectively from the delignified sclereids amounting to about 60% after 51 hours incubation. The combined action of xylanase and avicelase brought about a xylan degradation of about 70%. Addition of avicelase to the initially xylanase-treated material resulted in the degradation of cellulase up to 25%. Electron microscopy of the variously treated samples showed the micromorphological changes effected and gave an indication of the topochemical distribution of xylan and cellulose. Sulfuric acid treatment removed wall components from all the lamellae of the sclereid wall, showing no definite pattern. Xylanase effects an intense decrustation of wall material both at the lumen boundary as well as near to the middle lamella, whereby the pattern of degradation is irregular; the cellulose fibrils also become well exposed. The addition of avicelase to xylanase-treated sclereid holocellulose creates an increase in the degradation, which is especially localized in the lamellated wall near to the middle lamella/primary wall region and at the lumen boundary. There appears to be a total hydrolysis of both matrix and fibrillar substances, characteristically more in the lamellae with longitudinal bow-shaped fibrils. Based on these results it is concluded that there appears to be no definite differential distribution pattern of xylan in the two lamellae. The higher contrast in the lamellae with transversely oriented fibrils is interpreted as resulting from the packing density of cellulose fibrils. [ABSTRACT FROM AUTHOR]