Increase in in vivo Nitrate Reductase Activity in Maize Leaves in the Presence of Ammonium Pratima Mehata

Abstract

Summary Some aspects of enhancement in nitrate reductase activity in the presence of ammonium in maize leaves were examined. Floating maize leaf segments in light on either 10 mM KN0 3 or 5 mM(NH 4 ) 2 SO 4 caused, after a log of 1 to 3 h respectively, a gradual increase in nitrate reductase activity which continued up to 24 h. Several other ammonium salts had a similar effect increase in enzyme activity either in the presence of nitrate or ammonium was stimulated by the supply of molybdate up to 20 µand was inhibited by tungstate. Chloramphenicol in a concentration range of 0 to 500 µg ml- 1 had no effect on increase in enzyme activity either in the presence of nitrate or ammonium. During a 30 min assay of enzyme activity without supplying substrate (NO 3 - ) in the assay medium, the leaf segments pretreated with either CaC 12 or (NH 4 )2SO 4 produced little nitrite while those treated with KNO 3 produced appreciable nitrite. When 5 mM (NH 4 )2SO 4 was supplied together with various concentrations of nitrate, the enzyme activity increased considerably (over nitrate alone), but the increase at lower concentrations of nitrate were greater than those at higher concentrations. Possible alternative mechanisms of the increase in nitrate reductase activity in the presence of ammonium are discussed.