Nitrogen dynamics during O₃-induced accelerated senescence in hybrid poplar

Experiments were conducted to determine the fate of nitrogen (N) remobilized as a result of ozone (O₃)-induced accelerated senescence in hybrid poplar subjected to declining N availability concurrent with O₃ stress. Cuttings were grown in sand culture where the supply of N to the plant could be controlled on a daily basis and reduced in half of the plants when desired. Plants all initially received 3.57 mM N daily until approximately the 20 leaf stage after which daily supply of N was reduced to 0.71 mM. Plants were grown in open-top chambers in the field (Rock Springs, PA, USA) and received charcoal-filtered air, half also received supplemental O₃ to a level of 0.08 μL L^-1. Allocation of newly acquired N was determined with ¹⁵N. The specific allocation (mg labelled N mg^-1 total N) of labelled N to upper, expanding leaf N was not affected by O₃, but was strongly affected by N treatment. However, O₃ increased the relative partitioning of labelled N to the expanding leaves and the roots. The balance between partitioning of newly acquired N to the upper leaves and roots was not affected by O₃, but was reduced by N withdrawal. Calculated net N flux was strongly negative in the lower leaves of O₃-exposed, N withdrawal plants. Nitrogen uptake was not reduced by O₃. The allometric relationships between the roots and shoots were not affected by O₃ or N availability. The relative contribution of newly acquired versus remobilized N to new growth appears to be determined by N supply. Ozone exposure alters the allocation of newly acquired N via alterations in plant size, whereas N availability exerts a strong effect upon both plant size and N allocation.