Wood Chemical Composition in Species of Cactaceae: The Relationship between Lignification and Stem Morphology

<div><p>In Cactaceae, wood anatomy is related to stem morphology in terms of the conferred support. In species of cacti with dimorphic wood, a unique process occurs in which the cambium stops producing wide-band tracheids (WBTs) and produces fibers; this is associated with the aging of individuals and increases in size. Stem support and lignification have only been studied in fibrous tree-like species, and studies in species with WBTs or dimorphic wood are lacking. In this study, we approach this process with a chemical focus, emphasizing the role of wood lignification. We hypothesized that the degree of wood lignification in Cactaceae increases with height of the species and that its chemical composition varies with wood anatomy. To test this, we studied the chemical composition (cellulose, hemicellulose, and lignin content) in 13 species (2 WBTs wood, 3 dimorphic, and 8 fibrous) with contrasting growth forms. We also analyzed lignification in dimorphic and fibrous species to determine the chemical features of WBTs and fibers and their relationship with stem support. The lignin contents were characterized by Fourier transform infrared spectroscopy and high performance liquid chromatography. We found that 11 species have a higher percentage (>35%) of lignin in their wood than other angiosperms or gymnosperms. The lignin chemical composition in fibrous species is similar to that of other dicots, but it is markedly heterogeneous in non-fibrous species where WBTs are abundant. The lignification in WBTs is associated with the resistance to high water pressure within cells rather than the contribution to mechanical support. Dimorphic wood species are usually richer in syringyl lignin, and tree-like species with lignified rays have more guaiacyl lignin. The results suggest that wood anatomy and lignin distribution play an important role in the chemical composition of wood, and further research is needed at the cellular level.</p></div

Assignment of FTIR absorption bands in wood lignins.

<p>Assignment of FTIR absorption bands in wood lignins.</p

Chemical composition of lower wood in mature individuals of thirteen species of Cactaceae.

<p>The contents are reported in the total dry weight percent (% w/w). The standard deviation in each case was less than 10% on average.</p><p>(<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0123919#pone.0123919.s004" target="_blank">S1 Table</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0123919#pone.0123919.s003" target="_blank">S3 Dataset</a>. Supporting information for the S/G ratios)</p><p>* Species where the content of lignin in the lower wood was different between adult individuals, the difference in the lignin content in adults of F. pilosus was ≈ 19% and in E. platyacanthus was ≈ 23%, the other components of the wood vary proportionately. Moreover, in these dimorphic species occurs a change from WBTs wood in juvenile stages to fibrous wood in the mature ones.</p><p>Chemical composition of lower wood in mature individuals of thirteen species of Cactaceae.</p

Relationship between plant species size and percentage of syringyl lignin.

<p>Each dot represents the species size given in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0123919#pone.0123919.s005" target="_blank">S2 Table</a>.</p

Characteristics of individuals studied from thirteen species of Cactaceae, the regions of wood that were studied are indicated.

<p><b>Abbreviations.</b> Tuberculated (T), ribbed (R), tuberculated ribs (TR). Lower wood (LW): near to the vascular cambium (LWc), near to the pith (LWp). Upper wood (UW).</p><p>* In these dimorphic species occurs a change from WBTs wood in juvenile stages to fibrous wood in the mature ones.</p><p>Characteristics of individuals studied from thirteen species of Cactaceae, the regions of wood that were studied are indicated.</p

Fibrous wood in species of Cactaceae, cross sections, near the vascular cambium.

<p>Vessels embedded in a matrix of fibers with lignified rays except <i>Opuntia streptacantha</i>. (A) <i>Cylindropuntia imbricata</i>; (B) <i>Lophocereus marginatus</i>; (C) <i>Myrtillocactus geometrizans</i>; (D) <i>Opuntia streptacantha</i>: wood with vessels embedded in a matrix of fiber or parenchyma with intermixed WBTs and non lignified rays; (E) <i>Pereskia lychnidiflora</i>; (F) <i>Stenocereus dumortieri</i>. Bar is 200 μm; r = ray.</p

Non fibrous wood in species of Cacteae, cross sections near the vascular cambium.

<p>(A) <i>Ariocarpus retusus</i>: less lignified wood with vessels embedded in a matrix of WBTs separated by non lignified dilated rays; (B) <i>Coryphantha clavata</i>: wood with vessels embedded in a matrix of WBTs and narrow non lignified rays; (C) <i>Echinocactus platyacanthus</i>: wood with vessels embedded in a matrix of fibers and axial parenchyma with wider lignified rays; (D) <i>Ferocactus hamatacanthus</i>: wood with vessels embedded in a matrix of WBTs and fibers in similar proportions and a few narrow non lignified rays; (E) <i>Ferocactus pilosus</i>: wood with vessels embedded in a matrix of WBTs and lignified rays near the pith; (F) <i>Ferocactus pilosus</i>: wood with vessels embedded in a matrix of fibers and non lignified rays near the vascular cambium. Bar is 550 μm in A, B; 200 μm in C, E, F; 100 μm in D; r = ray.</p