Evidence of the late lignification of the G-layer inSimaroubatension wood, to assist understanding how non-G-layer species produce tensile stress

Abstract

To recover verticality after disturbance, angiosperm trees produce 'tension wood' allowing them to bend actively. The driving force of the tension has been shown to take place in the G-layer, a specific unlignified layer of the cell wall observed in most temperate species. However, in tropical rain forests, the G-layer is often absent and the mechanism generating the forces to reorient trees remains unclear. A study was carried out on tilted seedlings, saplings and adult Simarouba amara Aubl. trees-a species known to not produce a G-layer. Microscopic observations were done on sections of normal and tension wood after staining or observed under UV light to assess the presence/absence of lignin. We showed that S. amara produces a cell-wall layer with all of the characteristics typical of G-layers, but that this G-layer can be observed only as a temporary stage of the cell-wall development because it is masked by a late lignification. Being thin and lignified, tension wood fibres cannot be distinguished from normal wood fibres in the mature wood of adult trees. These observations indicate that the mechanism generating the high tensile stress in tension wood is likely to be the same as that in species with a typical G-layer and also in species where the G-layer cannot be observed in mature cells.

Tree growth stress and related problems

“…Simarouba amara (Simaroubaceae) is a typical example of those species [59]. By microscopic observations, Roussel and Clair observed that S. amara does produce a G-layer only at a temporary stage of cell wall development; however, it was masked by lignin deposited at a more advanced stage [59].…”

“…Ruelle et al reported that, in the tension wood of S. amara, the orientation of cellulose microfibrils is nearly parallel to the fiber axis, and its crystalline size is also significantly higher like in tension wood of Eperia falcataria, a tropical species producing a typical G-layer [60]. With reference to Ruelle et al, Roussel and Clair consider that the mechanism which generates the high tensile stress in tension wood of S. amara is likely to be the same as in species showing a typical unlignified G-layer [59]. wall.…”

“…wall. UV absorbance in the secondary wall gradually decreases with an increase of tensile growth stress [57] that once a high tensile stress has been generated in the cellulosic network of the G-layer, late lignification does not significantly modify this pre-existing tension [59,60].…”

Critical review on the mechanisms of maturation stress generation in trees

“…TW G-layers have high cellulose content and no lignin or are later lignified during the maturation process in some species but the level of lignification has been shown to be qualitatively lower in lignified G-layers [41]. Regarding CW, complete reviews are already available [54].…”

Effect of exogenous IAA on tension wood formation by facilitating polar auxin transport and cellulose biosynthesis in hybrid poplar (Populus deltoids × Populus nigra) wood

“…Therefore, TW is an attractive experimental system for exploring the development and the biochemical pathways of the secondary cell wall formation, as well as the mechanism underlying the induction and the formation of TW. Different stimuli are known to induce TW formation, especially the gravitational and hormonal stimuli (Jin et al 2014;Gerttula et al 2015;Gritsch et al 2015;Roussel and Clair 2015). Auxins mediate seasonal changes of cambial activity as a positional signal (Uggla et al 1996(Uggla et al , 1998 and, probably, they also play a key role in the control of TW formation (Morey and Cronshaw 1968a;Du et al 2004).…”