title: Onset of sheath extension and duration of lamina extension are major determinants of the response of maize lamina length to plant density.
abstract: OBJECTIVE
  Plants regulate their architecture strongly in response to density, and there is evidence that this involves changes in the duration of leaf extension. This questions the approximation, central in crop models, that development follows a fixed thermal time schedule. The aim of this research is to investigate, using maize as a model, how the kinetics of extension of grass leaves change with density, and to propose directions for inclusion of this regulation in plant models.
  
  
  METHODS
  Periodic dissection of plants allowed the establishment of the kinetics of lamina and sheath extension for two contrasting sowing densities. The temperature of the growing zone was measured with thermocouples. Two-phase (exponential plus linear) models were fitted to the data, allowing analysis of the timing of the phase changes of extension, and the extension rate of sheaths and blades during both phases.
  
  
  RESULTS
  The duration of lamina extension dictated the variation in lamina length between treatments. The lower phytomers were longer at high density, with delayed onset of sheath extension allowing more time for the lamina to extend. In the upper phytomers--which were shorter at high density--the laminae had a lower relative extension rate (RER) in the exponential phase and delayed onset of linear extension, and less time available for extension since early sheath extension was not delayed.
  
  
  CONCLUSIONS
  The relative timing of the onset of fast extension of the lamina with that of sheath development is the main determinant of the response of lamina length to density. Evidence is presented that the contrasting behaviour of lower and upper phytomers is related to differing regulation of sheath ontogeny before and after panicle initiation. A conceptual model is proposed to explain how the observed asynchrony between lamina and sheath development is regulated.
authors.affiliation: Institut National de la Recherche Agronomique, Unité Environnement et Grandes Cultures, Thiverval-Grignon, France. Bruno.Andrieu@grignon.inra.fr
authors.forename: Bruno
authors.initials: B
authors.lastname: Andrieu
authors.forename: Jonathan
authors.initials: J
authors.lastname: Hillier
authors.forename: Colin
authors.initials: C
authors.lastname: Birch
mesh_terms: D007700:Kinetics; D018520:Plant Shoots / Q000254:growth & development; D003313:Zea mays / Q000254:growth & development*