NITROGEN OXIDES PRODUCED DURING CO₂ ENRICHMENT III. EFFECTS ON TOMATO AT DIFFERENT PHOTON FLUX DENSITIES

Seedlings of Lycopersicon esculentum (tomato) were subjected to CO₂ -enriched air (1000 μl 1^-1 ) containing 1.5 μ l^-1 nitrogen oxides (NO_X ) for 25 d at four photon flux densities (30, 95, 175 and 250 μmol m^-2 s^-1 PAR). CO₂ enrichment without NO_x significantly increased the dry weights (47 to 93 %) at all light levels. Addition of NO_x strongly reduced the mean dry weight at the lowest light level even below that of the unenriched control. At the two highest light levels, NO_x reduced the dry weight, but much less than at the lowest level. NO_X caused severe leaf injury at the lowest light level, but this effect disappeared with increased photon flux density. A system was constructed for measurement of the net CO., exchange rate (CER) for single plants. Short-term measurements showed significant reductions of CER when the NO concentration was increased from 0 to 9μl 1^-1 at 550μmol m^-2 s^-1 but almost no effect at 140 to 200 μmol m^-2 s^-1 . NO caused similar percentage reductions of CER at 335 and 1000 μl ¹ CO₂ . The absorption of NO was not significantly affected by increasing the photon flux density from 150 to 550μmol m-² s^-2 .