Effects of Supplemental Nitrate Application on the Activity of Some Nitrogen Assimilation Enzymes and Leaf Tissue Productivity in Maize Seedlings

Maize (Zea mays L. cv. Golden Cross Bantam T51) seedlings were grown in a greenhouse at near optimal temperature (30/25°C) with seven levels of nitrate (0.4 to 12 mM). The activity of nitrate reductase, nitrite reductase, glutamine synthetase, GOGAT, and NADH-glutamate dehydrogenase in leaf tissues expressed on a fresh weight basis increased with increasing N except in the case of NADH-glutamate dehydrogenase where the activity continued to decrease. When the activity was expressed on a unit of soluble protein basis it was possible to classify these enzymes into three groups based on their response to the nitrate supplied. The activities of GOGAT in addition to nitrate reductase as the first group increased logarithmically with increasing nitrate application, in showing a concentration lag. Glutamine synthetase and nitrite reductase as the second group responded poorly to changes in the nitrate level. In contrast, the activity of NADH-glutamate dehydrogenase as the third group decreased exponentially with increasing N application. Correlation between leaf biomass induced by differences in the nitrate levels and enzyme activity was highly positive for nitrate reductase, glutamine synthetase, and GOGAT but highly negative for NADH-glutamate dehydrogenase. No significant correlation was found between the nitrite reductase activity and leaf biomass. The enhancement of the glutamine synthetase activity by the increase of N application expressed on a chlorophyll or packed volume basis was greater in bundle sheath strands than in mesophyll protoplasts.