Influence of nitrogen availability on ozone-induced accelerated foliar senescence

In order to determine if declines in nitrogen (N) availability during the growing season influence ozone (O3)-induced accelerated senescence, cuttings of hybrid poplar (Populus trichocarpa A. Henry x maximowizii Torr. & A. Gray clone '245') were grown in sand culture with N supplied daily at a constant concentration of 3.57 mM. When the plants had approximately 20 leaves the daily N supply was withdrawn in half of the plants to 0.71 mM. All plants were grown in open-top chambers receiving charcoal-filtered air; half also received supplemental O3 to a concentration of 0.08 mul l-1. The experiment was repeated in four consecutive summers, from 1996--1999. Experiments were also conducted to determine whether N remobilized as a result of O 3-induced accelerated senescence would supply a greater proportion of the N being incorporated into young leaves. A labeling experiment was performed, replacing the daily nutrient solution with one containing 15N to assess allocation of newly acquired N. It was concluded that a decline in N availability increases the rate of O3-induced accelerated senescence and that the response is associated with plant size and possibly continued active growth. Compensatory responses of young leaves to O3 exposure only occurred when N available to the plant declined and O3-induced accelerated senescence was most severe. In this poplar clone, the relative contribution of newly acquired versus remobilized N to new growth appears to be determined by N supply. O3 exposure alters allocation of newly acquired N via alterations in plant size, while N availability exerts a strong effect upon both plant size and N allocation.