Leaf and stem structure of poplar (Populus ×euramericana) as Influenced by O₃, NO₂, their combination, and different soil N supplies

Although increasing tropospheric ozone (O 3 ) concentrations as well as precursor NO 2 emissions and N deposition have been observed, the combination of their effects on deciduous trees is little understood. We therefore examined the growth and leaf injury response of a model tree (Populus ×euramericana (Dode) Guinier cuttings exposed before flush and until they reached a height of more than 1 m) to low and high soil N supply (105 or 315 mg N•L −1 substrate volume), to filtered air, and to filtered air with NO 2 (sinusoidal daily course with a mean of 100 nL•L −1 ), with O 3 (60 nL•L −1 ), or with a combination of both in climate-controlled chambers. High soil N increased total plant dry weight, leaf area, and xylem radius in plants fumigated with or without added NO 2 or O 3 . The number of leaves increased with high soil N independent of added NO 2 . The stomatal density was influenced by soil N and by fumigations, but the appearance of leaf injury symptoms, leaf loss, specific leaf weight, and bark radius were not modified by the soil N regimes. NO 2 alone, though applied in a sixfold ambient concentration, did not significantly increase plant growth. NO 2 and O 3 alone had opposite effects on specific leaf dry weight, stomatal density, and in the high fertilization regime, on the bark radius. The decrease in specific leaf dry weight and the appearance of early leaf symptoms were enhanced by NO 2 added to O 3 . Visible leaf injury caused by O 3 increased in parallel with microscopic changes in mesophyll cell walls, in the starch and protein patterns of mesophyll cells, in the bark cell content, and in the phloem sieve pores. NO 2 enhanced the negative effect of O 3 rather than compensated for a low soil N supply.