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Differences in the wood structural chemical composition in Cactaceae
The wood chemical composition has been studied mainly for hardwood species of economic importance, but it has been poorly documented with a structural approach in succulent species such as Cactaceae. The diversity in the wood types of cacti is related with their growth form, but there is little information about their relation with their chemical composition. So the aim of this study was to evaluate the wood chemical composition of different species representing the Cactaceae family and relate it with its anatomical structure through light and fluorescence microscopy. We used 33 species of Cactaceae mainly with non-fibrous wood. Just the vascular cylinder was removed and then, successive extractions were carried out based on the TAPPI Standard T207 om-93 to quantify and obtain the percentages of extractives and extractive free wood. Klason lignin, cellulose and hemicelluloses were also quantified. Anatomical sections were made and stained with safranin-fast green to characterize the tracheary elements and then to identify the differences in the chemical composition of the tracheary elements and fibers for each species with the fluorescence microscope. The results show that there are significant differences mainly between fibrous and non-fibrous species, with Kruskal Wallis analysis. In addition, the principal component analysis identified that the main factor that differentiates the species is lignin; the species are grouped based on their wood type (fibrous, non-fibrous, dimorphic) being homogenous among non-fibrous wood with a great variation among the species with dimorphic and fibrous wood. Fluorescence microscopy allowed visualizing differences among wide-band-tracheids, vessel elements and fibers, given mainly by the type of lignin that constitutes each cell type. The vessels have mainly guaiacyl-lignin, while fibers and wide-band-tracheids have syringyl lignin.