LEAF AND STEM STRUCTURE OF POPLAR (POPULUS X EURAMERICANA) AS INFLUENCED BY O-3, NO2, THEIR COMBINATION, AND DIFFERENT SOIL N SUPPLIES

Although increasing tropospheric ozone (O-3) concentrations as well as precursor NO2 emissions and N deposition have been observed, the comb ination of their effects on deciduous trees is little understood. We t herefore examined the growth and leaf injury response of a model tree (Populus x euramericana (Dode) Guinier cuttings exposed before flush a nd 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 NO2 (sinusoidal daily course with a mean of 1 00 nL . L(-1)), with O-3 (60 nL . L(-1)), or with a combination of bot h in climate-controlled chambers. High soil N increased total plant dr y weight, leaf area, and xylem radius in plants fumigated with or with out added NO2 or O-3. The number of leaves increased with high soil N independent of added NO2. 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. NO2 alone, though applied in a sixfold ambient concent ration, did not significantly increase plant growth. NO2 and O-3 alone had opposite effects on specific leaf dry weight, stomatal density, a nd 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, added to O-3. Visible leaf injury caused by O-3 i ncreased in parallel with microscopic changes in mesophyll cell walls, in the starch and protein patterns of mesophyll cells, in the bark ce ll content, and in the phloem sieve pores. NO2 enhanced the negative e ffect of O-3 rather than compensated for a low soil N supply.