EFFECTS OF NO2 AND NITRATE ON SULFATE ASSIMILATION IN MAIZE

The effect of nitrogen dioxide (NO2) and nitrate on sulfate assimilati on was studied in maize seedlings. The seedlings (Zea mays L. cv. LG 9 ) were grown on a N-free nutrient solution for 10 days and subsequentl y fumigated with 520 nLL(-1) NO2 or transferred to a nutrient solution containing 4 mM nitrate. Fresh weight, contents of protein, cysteine, gamma-glutamylcysteine and glutathione, and the extractable activity of adenosine 5'-phosphosulfate sulfotransferase, a key enzyme of sulfa te assimilation, were measured in the second, third and fourth leaves during the following 7 days. For comparison, the activity of nitrate r eductase was measured. The level of extractable proteins was higher in the leaves of fumigated or nitrate treated seedlings than in leaves o f controls without NO2 and nitrate. Adenosine 5/-phosphosulfate sulfot ransferase and nitrate reductase activities of the second and third le aves increased to a significantly higher level after 1 day of NO2 fumi gation or nitrate treatment. Nitrate induced a significantly higher le vel of glutathione in the leaves, whereas its level at NO2 fumigation was not always significantly higher than that of the controls. Even th ough the quantitative changes in enzyme activities induced by nitrate were much greater, our results, in principle, show the same qualitativ e effects of nitrate and NO2, indicating that NO2 was used as a N-sour ce and regulated sulfate assimilation in the same way as nitrate.