Nitrate Reductase Activity in Corn Leaves as Related to Yields of Grain and Grain Protein1

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Abstract
The objective was to determine whether a relationship between nitrate reductase activity (NRA) and grain yields and grain protein could be detected for six corn (Zea mays L.) hybrids grown under field conditions with supplemental N and irrigation. NRA was expressed as seasonal input of reduced N (kg N ha‐1) and as seasonal averages of activity (µmoles N, g fresh wt‐1, hr‐1). The treatment variables were plant density (59,300 and 79,000 plants ha‐1) and tune of application (14 days after tassel initiation, at tassel emergence, and at silking) of nitrate‐N (337 kg N ha‐1).Each supplemental N treatment, regardless of time of application, significantly increased the percent of grain protein for all genotypes and amount of grain protein ha‐1 for all but one hybrid (No. 3). In contrast, the supplemental N treatments did not cause significant increases in grain yields within a given genotype. There were significant differences in grain yield and grain protein among the genotypes. The treatments increased leaf blade nitrate concentration which resulted in a corresponding increase in NRA. However, each successive N application was less effective in increasing and maintaining leaf nitrate content and enzyme activity. None of the hybrids were able to maintain a high concentration of leaf blade nitrate after silking, even though massive amounts of nitrate were applied to the soil. Compared to all other hybrids, hybrid No. 4 maintained a slightly higher level of nitrate concentration and enzyme activity in the leaves during the post‐anthesis stage.NRA of the total leaf canopy, expressed as seasonal averages or converted into seasonal input of reduced N, showed a significant positive correlation with grain protein (kg N/ha), grain yield, and total reduced N in the vegetative material (above ground) and grain and stover at maturity. The highest correlations between NRA and yields of grain and grain protein (kg N/ha) were obtained during the stages of ear initiation and development. This suggests that a minimal number (1 to 3) of samplings would be as effective as the laborious full season samplings (12 or more dates) in selecting individual plants or varieties that have a high potential for grain yields or grain protein production.

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