Temporal and spatial regulation of nitrate reductase and nitrite reductase in greening maize leaves

The temporal and spatial regulation of nitrate reductase (NR), a cytosolic enzyme, and nitrite reductase (NIR), a chloroplastic enzyme, was examined in first leaf of maize seedlings. The induction of NR and NIR activity showed a biphasic response with reference to exogenous concentration of nitrate, which probably resulted from the biphasic uptake of nitrate in seedlings. The time course of accumulation of NR and NIR activities in maize leaf followed a pattern that was loosely co-ordinated, with NR activity peaking on the sixth day after sowing and NIR activity peaking on the seventh day. Although the magnitudes of induction in intact and detached leaves were different, the profiles of NR and NIR induction were basically similar. A study of NR and NIR distribution along the length of maize leaf showed opposite profiles of distribution, with NR activity being maximal at the base of the leaf, and NIR activity being maximal at the tip. These results suggest that the temporal and spatial regulation of NR and NIR are not tightly co-regulated in maize leaves. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.