EFFECTS OF NO (+NO2) POLLUTION ON GROWTH, NITRATE REDUCTASE ACTIVITIES AND ASSOCIATED PROTEIN CONTENTS IN GLASSHOUSE LETTUCE GROWN HYDROPONICALLY IN WINTER WITH CO2 ENRICHMENT

Winter hydroponic growth of several lettuce cultivars under glass show ed considerable inhibition (up to 47%) of growth after 6 wk exposure t o concentrations of NO (+ NO2; 450 nl l(-1) in total) typical of emiss ions from propane burners used for direct heating and CO2 enrichment. After a further 4 wk under similar conditions, however, these growth d epressions were replaced by a swing into benefit so that, by harvest, pollutant-exposed lettuces were bigger and had faster assimilation rat es than those growing in clean CO2-enriched air. This adaptation may p artly be explained by enhanced use of NO2-derived N by lettuce leaves, a consequence of increased nitrate reductase (NaR) activities and amo unts of associated NaR proteins, despite adequate nitrate also being a vailable in the hydroponic fluid. Rates of NaR activity in the roots, by contrast, were depressed by NO (+ NO2) pollution. NaR activities we re highest in early afternoon in clean or polluted air but these daily patterns did not coincide with the content of NaR-associated proteins determined by ELISA. Other mechanisms of modulating NaR activity must therefore be responsible.