RESPONSE OF NITROGEN-METABOLISM IN BEANS (PHASEOLUS-VULGARIS L) AFTER EXPOSURE TO OZONE AND NITROGEN-DIOXIDE, ALONE AND IN SEQUENCE

Bush bean (Phaseolus vulgaris L.) plants were exposed to low levels of ozone (O3) and/or nitrogen dioxide (NO2) in open-top chambers during the growing seasons of 1988 and 1989. Treatments consisted of charcoal-filtered (CF) air, and CF air enriched with either O3 (50-60 nl l-1), NO2 (30-40 nl l-1) or both gases. A daily sequential exposure, O3 followed by NO2, was used in each year in the combined treatment: O3 was added for 8 h d-1 from 08.00 h until 16.00 h, and NO2 for 16 h d-1 from 16.00 h until 08.00 h. Growth variables and key enzymes of N assimilation in leaves were investigated during vegetative growth and at anthesis. Pollutant effects varied between years. No significant effects were found in 1988. In 1989 NO2, alone or in sequential exposure with O3, increased leaf dry weight and total biomass until anthesis. Moreover, there was a parallel increase in the extractable activity of both nitrate and nitrite reductase in the NO2 treatments during vegetative growth, while glutamine synthetase and glutamate dehydrogenase were only increased by sequential exposure to O3 + NO2. In contrast, during anthesis the activities of nitrite reductase and glutamine synthetase were lowest in leaves sequentially exposed to O3 + NO2. Ozone alone had very little effect on N metabolism but suppressed growth during anthesis. At pod maturity, the lowest leaf dry weight and leaf area occurred in plants exposed to the sequential combination of O3 + NO2, but yield (pod weight) was not significantly affected by any of the pollutant treatments. It is concluded that chronic exposure especially to the sequence O3-NO2 reduced the capacity of the plants for N assimilation. The observed shift in nitrogen metabolism during the plants' development may have contributed to the adverse effects of the sequential treatment O3 + NO2 on growth variables at the end of the exposure period.