Effects of NO 2 and Nitrate on Sulfate Assimilation in Maize

Abstract

The effect of nitrogen dioxide (NO 2 ) and nitrate on sulfate assimilation 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 subsequently fumigated with 520 nLL -1 NO 2 or transferred to a nutrient solution containing 4 mM nitrate. Fresh weight, contents of protein, cysteine, γ-glutamylcysteine and glutathione, and the extractable activity of adenosine 5'-phosphosulfate sulfotransferase, a key enzyme of sulfate assimilation, were measured in the second, third and fourth leaves during the following 7 days. For comparison, the activity of nitrate reductase was measured. The level of extractable proteins was higher in the leaves of fumigated or nitrate treated seedlings than in leaves of controls without NO 2 and nitrate. Adenosine 5'-phosphosulfate sulfotransferase and nitrate reductase activities of the second and third leaves increased to a significantly higher level after 1 day of NO 2 fumigation or nitrate treatment. Nitrate induced a significantly higher level of glutathione in the leaves, whereas its level at NO 2 fumigation was not always significantly higher than that of the controls. Even though the quantitative changes in enzyme activities induced by nitrate were much greater, our results, in principle, show the same qualitative effects of nitrate and NO 2 , indicating that NO 2 was used as a N-source and regulated sulfate assimilation in the same way as nitrate.