Alok had carved his name in the trunk of a tree at a height of 2 metres from the ground level. When he visited the same tree after 2 years, did he notice any change in the position of his curved name? Why?
Answers
In tree trunks, the motor of gravitropism involves radial growth and differentiation of reaction wood (Archer, 1986). The first aim of this study was to quantify the kinematics of gravitropic response in young poplar (Populus nigra x Populus deltoides, ‘I4551’) by measuring the kinematics of curvature fields along trunks. Three phases were identified, including latency, upward curving, and an anticipative autotropic decurving, which has been overlooked in research on trees. The biological and mechanical bases of these processes were investigated by assessing the biomechanical model of Fournier et al. (1994). Its application at two different time spans of integration made it possible to test hypotheses on maturation, separating the effects of radial growth and cross section size from those of wood prestressing. A significant correlation between trunk curvature and Fournier's model integrated over the growing season was found, but only explained 32% of the total variance. Moreover, over a week's time period, the model failed due to a clear out phasing of the kinetics of radial growth and curvature that the model does not take into account. This demonstrates a key role of the relative kinetics of radial growth and the maturation process during gravitropism. Moreover, the degree of maturation strains appears to differ in the tension woods produced during the upward curving and decurving phases. Cell wall maturation seems to be regulated to achieve control over the degree of prestressing of tension wood, providing effective control of trunk shape.
Gravitropic movements have been observed on a large range of herbaceous species as well as woody plants. Two different motors enabling axis curvature exist: in zones of primary elongation growth, the motor of reorientation is differential growth; the growth is stimulated, preferentially on one side of the organ (Barlow et al., 1989; Cosgrove, 1997); in zones where elongation is completed but radial growth still active, stem curvature occurs by differential maturation between the two sides of the organ.
Maturation strains appear because of dimensional changes of the cells during the maturation of the secondary cell wall (Fournier et al., 1994). The dimensional changes can be either shrinkage or swelling, depending on the direction, wood type, and species. In woody angiosperm species such as poplar (Populus spp.), maturation strains involve shrinkage in the longitudinal and radial directions, and swelling in the tangential direction (Archer, 1986). Maturation strains in the longitudinal direction are about 10 times higher than in the other directions (Archer, 1986). Tree axes are generally slender structures and can be considered as beams. From a mechanical point of view, tropic curvature is mainly due to the effect of longitudinal maturation strains. However, these strains are constrained within the trunk. Indeed, inner layers of older wood to which differentiating cells are stuck resist this deformation because of their mechanical rigidity. A mechanical equilibrium in which maturation stresses are produced is achieved (Fournier et al., 1994). Peripheral, differentiating wood cells undergo longitudinal tension because their maturation strains are resisted and compress the inner tissues. If the amount of inner tissue is large enough compared with the thickness of the maturation zone, the rigidity of the inner core resists, developing locked-in maturation strains. Large maturation stresses are generated near the periphery.
The maturation stresses can be measured indirectly: The internal mechanical state is locally disrupted by making cuts or drilling a hole in the wood after removal of the bark. Disruption of the internal mechanical equilibrium enables the locked-in maturation strains to be released. The wood that was under tension within the standing tree subsequently shrinks. The longitudinal shrinkage is then measured. It has been be observed that the measured residual longitudinal maturation strains (rlms) underestimate the original maturation strain for two reasons: The piece of wood on which shrinkage is measured is not completely disconnected from the rest of the trunk (Coutand et al., 2004); green wood is known to be a viscoelastic material so that the release of some of the initial strains is delayed and can only be released after a long time or after an accelerating hygrothermal treatment (Gril and Thibaut, 1994). Maturation strains are generally higher (in absolute values) in reaction wood than in opposite or normal wood so that any asymmetric production of reaction wood induces a reorientation of the stem (Archer, 1986).
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