Short-term physiological and developmental responses to nitrogen availability in hybrid poplar.

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 x 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 x deltoides exhibits in response to differential N availability.