The Gravitropic Response of Poplar Trunks: Key Roles of Prestressed Wood Regulation and the Relative Kinetics of Cambial Growth versus Wood Maturation

Abstract

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.

Biomechanical design and long-term stability of trees: Morphological and wood traits involved in the balance between weight increase and the gravitropic reaction

“…The model allows the role played by the different components (size described by diameter and height, allometric relations between mass, height and diameter, wood quality and variations within the cross-section) to be studied. The maturation strain asymmetry is clearly the main motor of the reaction, so that the strong assumptions of simple models used in previous studies (Fournier et al, 1994a;Coutand et al, 2007) are justified a posteriori. However, other aspects of growth asymmetry (in ring thickness and wood stiffness), contribute significantly to improve the curving process.…”

Unifying model of shoot gravitropism reveals proprioception as a central feature of posture control in plants

“…Number of replicates were 30 for wheat, 15 for Arabidopsis, and 5 for all the other species. Published data were also reprocessed from similar experiments on Impatiens glandilufera stems by Pfeffer (35) and on poplar trunks (Populus deltoides x nigra cv I4551) by Coutand et al (9). More precisely, two types of experiments were conducted, as explained in SI Appendix, section S2.2 and S2.5: (i) detailed kinematics experiments on two model species (Arabidopsis and wheat), based on time-lapse photography and quantitative analysis of curving-decurving kinematics (SI Appendix, sections S2.2 to S2.4) and (ii) simplified morphometric experiments on all the genotypes, to estimate the bending number (through B l = L gz /L c ) and the (transient) global mode M, defined as the maximum number of places below the apex where the tangent to the central line of the organ is vertical (SI Appendix, Fig.…”

“…As for all tropisms, a directional stimulus is sensed (gravity in this case), and the curvature of the organ changes over time until a set-angle and a steady-state shape are reached (2,7,8). The change in shape is achieved by differential elongation for organs undergoing primary growth (e.g., coleoptiles) or by differential differentiation and shrinkage of reaction wood for organs undergoing secondary growth (e.g., tree trunks) (9). Tropisms are complex responses, as unlike other plant movements (e.g., fast movements) (5, 10) the motor activity generated is under continuous biological control (e.g., refs.…”

“…In essence, this has been described as a biphasic pattern of general curving followed by basipetal straightening (GC/BS) (4,17). First, the organ curves up gravitropically, then a phase of decurving starts at the tip and propagates downward, so that the curvature finally becomes concentrated at the base of the growth zone and steady (7)(8)(9)(18)(19)(20). This decurving, which has also been described as autotropic (i.e., the tendency of plants to recover straightness in the absence of any external stimulus) (7,21), may start before the tip reaches the vertical (4).…”

A model to simulate the gravitropic response and internal stresses in trees, considering the progressive maturation of wood

“…In fact, a recent study on the kinematics of the gravitropic movement of artificially tilted poplars (Coutand et al, 2007) revealed three phases: a latency phase (no curving up of the tilted trunk), a gravitropic phase (curving up) and an autotropic phase (decurving enabling the trunk to become straight and vertical). Within this last phase TW is produced on the opposite side.…”