title: Stem extension and mechanical stability of Xanthium canadense grown in an open or in a dense stand.
abstract: OBJECTIVE
  Plants in open, uncrowded habitats typically have relatively short stems with many branches, whereas plants in crowded habitats grow taller and more slender at the expense of mechanical stability. There seems to be a trade-off between height growth and mechanical stability, and this study addresses how stand density influences stem extension and consequently plant safety margins against mechanical failure.
  
  
  METHODS
  Xanthium canadense plants were grown either solitarily (S-plants) or in a dense stand (D-plants) until flowering. Internode dimensions and mechanical properties were measured at the metamer level, and the critical buckling height beyond which the plant elastically buckles under its own weight and the maximum lateral wind force the plant can withstand were calculated.
  
  
  RESULTS
  Internodes were longer in D- than S-plants, but basal diameter did not differ significantly. Relative growth rates of internode length and diameter were negatively correlated to the volumetric solid fraction of the internode. Internode dry mass density was higher in S- than D-plants. Young's modulus of elasticity and the breaking stress were higher in lower metamers, and in D- than in S-plants. Within a stand, however, both moduli were positively related to dry mass density. The buckling safety factor, a ratio of critical buckling height to actual height, was higher in S- than in D-plants. D-plants were found to be approaching the limiting value 1. Lateral wind force resistance was higher in S- than in D-plants, and increased with growth in S-plants.
  
  
  CONCLUSIONS
  Critical buckling height increased with height growth due mainly to an increase in stem stiffness and diameter and a reduction in crown/stem mass ratio. Lateral wind force resistance was enhanced due to increased tissue strength and diameter. The increase in tissue stiffness and strength with height growth plays a crucial role in maintaining a safety margin against mechanical failure in herbaceous species that lack the capacity for secondary growth.
authors.affiliation: Department of International Agricultural Development, Tokyo University of Agriculture, Setagaya, Tokyo 156-8502, Japan.
authors.forename: Ryoji
authors.initials: R
authors.lastname: Watari
authors.affiliation: Department of International Agricultural Development, Tokyo University of Agriculture, Setagaya, Tokyo 156-8502, Japan.
authors.forename: Hisae
authors.initials: H
authors.lastname: Nagashima
authors.affiliation: Department of International Agricultural Development, Tokyo University of Agriculture, Setagaya, Tokyo 156-8502, Japan tadakihirose@gmail.com.
authors.forename: Tadaki
authors.initials: T
authors.lastname: Hirose
mesh_terms: D001696:Biomechanical Phenomena; D008954:Models, Biological; D018547:Plant Stems / Q000254:growth & development / Q000502:physiology*; D014919:Wind; D014934:Wood / Q000254:growth & development / Q000502:physiology; D031210:Xanthium / Q000254:growth & development / Q000502:physiology*
keywords: Allometry; Xanthium canadense; Young's modulus of elasticity; biomechanics; breaking stress; buckling safety factor; cocklebur; internode; metamer; plant height; stand density; stem extension; wind force resistance