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WHOLE PLANT AND ECOPHYSIOLOGY

The Gravitropic Response of Poplar Trunks: Key Roles of Prestressed Wood Regulation and the Relative Kinetics of Cambial Growth versus Wood Maturation^[C],[OA]

Unité Mixte de Recherche 547 Physiologie Intégrée de l'Arbre Fruitier et Forestier, Institut National de la Recherche Agronomique, Université Blaise Pascal, 63100 Clermont-Ferrand, France (C.C., B.M.); and Ecole Nationale du Génie Rural et des Eaux et Forêts, Unité Mixte de Recherche Ecologie des Forêts de Guyane, 97310 Kourou, France (M.F.)

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.

^[C] Some figures in this article are displayed in color online but in black and white in print.

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www.plantphysiol.org/cgi/doi/10.1104/pp.106.088153

^* Corresponding author; e-mail coutand{at}clermont.inra.fr; fax 33–04–73–62–44–54.

Received August 11, 2006; accepted April 16, 2007; published April 27, 2007.

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