Cellular UV Microspectrophotometric Investigation of Sapelli Heartwood (Entandrophragma cylindricum Sprague) from Natural Provenances in Africa

Abstract

Summary The colour of Sapelli heartwood (Entandrophragma cylindricum Sprague) is variable depending on habitat. In recent years an unusual pinkish-brown colour has been observed in commercial sawn timber of this species from Central Africa. Initial studies of Sapelli heartwood samples from various provenances have furthermore demonstrated pronounced differences in its decay resistance. Heartwood extractive composition varies considerably between provenances. Samples of Sapelli heartwood from the Central African Republic (RCA) contained 6.4% by weight of extractives compared with 12.5% for those from Cameroon and four other provenances investigated. UV microspectrophotometric analysis of heartwood extractives was used to localise phenolic extractives within wood cell walls and to identify their distribution at tissue level. The S2 and S3 wall layers of vessels and fibres in the heartwood were discriminated from sapwood by their high concentration of non-lignin phenolic extractives. These extractives in the cell walls were also found to be variable. As UV absorption properties show, the vessel and fibre secondary walls of Cameroon heartwood had a considerably higher concentration of extractives in comparison with samples from the RCA. In sapwood, the UV absorption levels for the fibre S2 layer and the vessel S3 layer were independent of the extraction procedure, suggesting the phenolic extractives in this part of the trunk were in low concentrations. Furthermore it is suggested that polyphenols constitute part of the cell wall matrix. The non-extractable phenolic content was found to be independent of provenances as demonstrated by wood from Cameroon and the RCA. The natural durability of Sapelli heartwood against fungal decay appears to be dependent on the extractive composition and on their concentration in the fibre tissue.