Critical review on the mechanisms of maturation stress generation in trees

Abstract

Trees control their posture by generating asymmetric mechanical stress around the periphery of the trunk or branches. This stress is produced in wood during the maturation of the cell wall. When the need for reaction is high, it is accompanied by strong changes in cell organization and composition called reaction wood, namely compression wood in gymnosperms and tension wood in angiosperms. The process by which stress is generated in the cell wall during its formation is not yet known, and various hypothetical mechanisms have been proposed in the literature. Here we aim at discriminating between these models. First, we summarize current knowledge about reaction wood structure, state and behaviour relevant to the understanding of maturation stress generation. Then, the mechanisms proposed in the literature are listed and discussed in order to identify which can be rejected based on their inconsistency with current knowledge at the frontier between plant science and mechanical engineering.

Plant ‘muscles’: fibers with a tertiary cell wall

“…Although gelatinous fibers are found in different taxonomic groups, they predominate in angiosperm plants. The best‐known example of fibers with a tertiary cell wall is G‐fibers of tension wood (Mellerowicz & Gorshkova, ; Alméras & Clair, ; Groover, ). However, based on compositional and architectural similarity, it appears that G‐fibers are not exclusive to trees and to xylem cell types (Gorshkova et al ., ; Tomlinson et al ., ).…”

“…Detachment of the G‐layer and its shrinkage as compared with the surrounding cell wall layers are often seen in cross‐sections of G‐fibers (Clair et al ., ), a further indication of tension in the tertiary cell wall. Arguments for the existence tension, its amount and the ways to measure it in reaction wood were recently summarized (Alméras & Clair, ). In all plant systems, the tertiary cell wall functions to provide tension that fosters spring‐like properties to help reduce mechanical load or to move particular plant parts.…”

Tree growth stress and related problems

“…The assumption of matrix swelling between bridged microfibrils [13,67], based on observed gel-like features of G-layer [96], was able to explain both the axial contraction and the transverse expansion of G-fibers. All are critically introduced in a recent review of the generation mechanisms of maturation stress [97].…”

Investigation of the Mechanical Properties of Flax Cell Walls during Plant Development: The Relation between Performance and Cell Wall Structure

“…The looser appearance of the Gn layer and its lower indentation modulus compared to the G layer argue for a mechanism of aggregation of cellulose microfibrils and entrapment of the matrix material, during the maturation of the cell wall from Gn to G layer. This maturation could induce a change in the cell wall structure that leads to changes in the mechanical properties, in parallel to the induction of the tensile stress [9,22]. However, when the fibers from the bottom part of the plant are considered (Figure 1c), although a thin Gn layer is still visible at this stage of growth, the major portion of the thickened cell wall is much more homogeneous and gives higher mechanical properties (an indentation modulus of 19.5 ± 2.9 GPa).…”