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

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.

Key Words: Biochemical criteria • Nitrogen metabolism • Genetic potential

² Assistant Professor, Agronomy Department, North Dakota State University, Fargo; Associate Professor of Plant Genetics, and Professor of Agronomy, University of Illinois, Urbana, Illinois 61801.

Received for publication October 25, 1972.