Responses of NADH and NADPH nitrate reductase activities and nitrate uptake to chlorate supply in maize seedlings

Abstract

Summary Experiments were conducted to examine the responses of NADH and NADPH specific nitrate reductase (NR) activities from various organs of maize seedlings to chlorate, with a view to understand the mechanism of chlorate toxicity and to ellucidate the significance of NAD(P)H: NR under toxic conditions. Supply of 1, 5, 10 or 20 mmol/L KCIO 3 under inducing conditions (10 mmol/L nitrate) to 4-5-day-old seedlings grown without nitrogen for 24 h in light (67 W m −2 ) at 25°C inhibited NADH:NR activity in the shoots and scutellum, and to some extent in the roots also. On the other hand, NADPH: NR activity was increased substantially in the scutellum and decreased (above 1 mmol/L chlorate) in roots by chlorate supply. Total protein content of the shoot or scutellum was inhibited by 20 mmol/L chlorate and was almost unaffected at lower concentrations in these organs; it increased slightly at all concentrations of chlorate in the roots. Nitrate acquisition by the seedlings was inhibited to an almost similar magnitude at each concentration of chlorate used. In another experiment involving short term (3h) supply of 5 mmol/L chlorate to the seedlings under inducing (+nitrate) or non-inducing (−nitrate) conditions, NADH:NR activity was usually inhibited and NADPH:NR activity was increased in scutellum or unaffected in roots. During in vitro incubation of enzyme preparations for 1 h at 4°C with 5 mmol/L chlorate, 10 mmol/L nitrate or chlorate + nitrate, NADH:NR activity increased in shoots and to some extent in roots also, but had variable effects in scutellum. Root NADPH:NR activity also increased by incubation with either of these ions, while in the scutellum it was drastically reduced. The experiments demonstrate an organ specific and isoform specific response of NR to chlorate and also that chlorate was acting directly on enzyme molecules under in vitro conditions.