The aims of this study were to determine the wood chemical composition of 25 species of Cactaceae and to relate the composition to their anatomical diversity. The hypothesis was that wood chemical components differ in relationship to their wood features. The results showed significant differences in wood chemical compounds across species and genera (P < 0.05). Pereskia had the highest percentage of lignin, whereas species of Coryphantha had the lowest; extractive compounds in water were highest for Echinocereus, Mammillaria, and Opuntia. Principal component analysis showed that lignin proportion separated the fibrous, dimorphic, and non-fibrous groups; additionally, the differences within each type of wood occurred because of the lignification of the vascular tissue and the type of wall thickening. Compared with other groups of species, the Cactaceae species with fibrous and dimorphic wood had a higher lignin percentage than did gymnosperms and Acer species. Lignin may confer special rigidity to tracheary elements to withstand desiccation without damage during adverse climatic conditions.

The xylem of Cactaceae is a complex system with different types of cells whose main function is to conduct and store water, mostly during the development of primary xylem, which has vessel elements and wide-band tracheids. The anatomy of primary xylem of Cactaceae has been widely studied, but little is known about its chemical composition. The aim of this study was to determine the structural chemical composition of the primary xylem of Cactaceae and to compare it with the anatomy in the group. Seeds from eight cacti species were used, representing the Pereskioideae, Opuntioideae, and Cactoideae subfamilies. Seeds were germinated and grown for 8 months. Subsequently, only the stem of the seedling was selected, dried, milled, and processed following the TAPPI T-222 om-02 norm; lignin was quantified using the Klason method and cellulose with the Kurshner–Höffer method. Using Fourier transform infrared spectroscopy, the percentage of syringyl and guaiacyl in lignin was calculated. Seedlings of each species were fixed, sectioned, and stained for their anatomical description and fluorescence microscopy analysis for the topochemistry of the primary xylem. The results showed that there were significant differences between species (p < 0.05), except in the hemicelluloses. Through a principal component analysis, it was found that the amount of extractive-free stem and hot water-soluble extractives were the variables that separated the species, followed by cellulose and hemicelluloses since the seedlings developed mainly parenchyma cells and the conductive tissue showed vessel elements and wide-band tracheids, both with annular and helical thickenings in secondary walls. The type of lignin with the highest percentage was guaiacyl-type, which is accumulated mainly in the vessels, providing rigidity. Whereas in the wide-band tracheids from metaxylem, syringyl lignin accumulated in the secondary walls S2 and S3, which permits an efficient flow of water and gives the plant the ability to endure difficult conditions during seedling development. Only one species can be considered to have paedomorphosis since the conductive elements had a similar chemistry in primary and secondary xylem.

Lignin is one of the most studied and analyzed materials due to its importance in cell structure and in lignocellulosic biomass. Because lignin exhibits autofluorescence, methods have been developed that allow it to be analyzed and characterized directly in plant tissue and in samples of lignocellulose fibers. Compared to destructive and costly analytical techniques, fluorescence microscopy presents suitable alternatives for the analysis of lignin autofluorescence. Therefore, this review article analyzes the different methods that exist and that have focused specifically on the study of lignin because with the revised methods, lignin is characterized efficiently and in a short time. The existing qualitative methods are Epifluorescence and Confocal Laser Scanning Microscopy; however, other semi-qualitative methods have been developed that allow fluorescence measurements and to quantify the differences in the structural composition of lignin. The methods are fluorescence lifetime spectroscopy, two-photon microscopy, Föster resonance energy transfer, fluorescence recovery after photobleaching, total internal reflection fluorescence, and stimulated emission depletion. With these methods, it is possible to analyze the transport and polymerization of lignin monomers, distribution of lignin of the syringyl or guaiacyl type in the tissues of various plant species, and changes in the degradation of wood by pulping and biopulping treatments as well as identify the purity of cellulose nanofibers though lignocellulosic biomass.

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Cactaceae family has heterogeneity in the accumulation of lignocellulose due to the diversity of shapes and anatomy of the wood. Most studies focus on fibrous and dimorphic species; but the non‐fibrous species are poorly studied. The aims of this work were to analyze the syringyl/guaiacyl ratio of lignin and its distribution in secondary xylem, especially in non‐fibrous species. The syringyl/guaiacyl (S/G) ratio was quantified from 34 species of cacti by nitrobenzene oxidation of free‐extractive wood. The distribution of lignocellulose in wood sections stained with safranin O/fast green was determined with epifluorescence microscopy. The S/G ratio was heterogeneous; most of the non‐fibrous species had a higher percentage of syringyl, while the fibrous ones accumulate guaiacyl. Fluorescence emission showed that vessel elements and wide‐band tracheids had similar tonalities. It is hypothesized that the presence of a higher percentage of syringyl in most cacti is part of the defense mechanism against pathogens, which together with the succulence of the stem represent adaptations that contribute to survival in their hostile environments.