A decline in nitrogen availability affects plant responses to ozone

The effects are presented here of a decline in nitrogen (N) availability on ozone (O₃)-induced accelerated foliar senescence during the growing season in a hybrid poplar. Cuttings of Populus trichocarpa x maximowizii were grown in sand culture where N supply to the plant could be controlled on a daily basis and reduced in half of the plants when desired. All plants received charcoal-filtered air; half also received supplemental O₃. Ozone and N withdrawal both reduced plant growth. Plants grown in both N treatments displayed leaf senescence and abscission in response to O₃, but leaf abscission in the N-withdrawal treatment was approximately double that of the constant-N treatment. Ozone had differential effects on light-saturated net photosynthesis (A_sat) and total soluble protein in the younger and older foliage depending on N treatment. A decline in N availability increased the rate of O₃-induced accelerated senescence and was associated with plant size and possibly continued active growth. Compensatory responses of young leaves to O₃ exposure only occured when N availability to the plant declined and O₃-induced accelerated senescence was most severe.