• bark storage protein;
  • carbon–nitrogen balance;
  • leaf development;
  • nitrogen;
  • pectin methylesterase inhibitor;
  • photosynthesis;
  • sylleptic branching


  • • 
    Nitrogen fertilization induces dramatic changes in the growth and development of plants, including forest trees. In this study we examined short-term responses of hybrid poplar, Populus balsamifera ssp. trichocarpa × deltoides, to N fertilization.
  • • 
    Glasshouse-grown saplings subjected to limiting, intermediate, and luxuriant levels of ammonium nitrate over a 28 d time course demonstrated rapid changes to whole-plant architecture and biomass accumulation. Nitrogen-associated shifts in allocation occurred in temporally distinct stages. Nitrogen availability modulated parameters that affect carbon gain, including light-saturated net photosynthesis and leaf area. These parameters were affected by N-induced changes to leaf maturation and senescence. Leaf area was also affected by N-induced sylleptic branch development.
  • • 
    Genes encoding vegetative storage proteins and starch biosynthetic enzymes exhibited contrasting patterns of expression under differential N availability. A gene encoding a previously uncharacterized putative pectin methylesterase inhibitor displayed expression patterns comparable to the starch biosynthetic genes.
  • • 
    The results of this study illustrate the phenotypic plasticity that P. balsamifera ssp. trichocarpa × deltoides exhibits in response to differential N availability.