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Seasonal and Canopy Variation in Nitrate Reductase Activity of Soybean (Glycine max L. Merr.) Varieties¹

An in vivo assay was used to determine if differences existed in nitrate reductase activity among 16 soybean (Glycine max L. Merr.) varieties within four maturity groups. Canopy and seasonal profiles of nitrate reductase activity were determined. All varieties exhibited highest nitrate reductase activity per leaf x nr in upper leaves of the plant canopy during growth stages up to mid pod fill. Activity levels were similar throughout the plant leal canopy following mid pod fill. Nitrate reductase activity per leaf x hr at a given node was generally highest the initial date at which the leaf was sampled and activity declined with leaf age. Mean nitrate reductase activity (µmoles NO-₂ formed per g fresh wt x hr) for the entire leaf canopy was highest in the seedling stage for all maturity groups and declined as the plants matured. Activity of the total plant (µmoles NO-₂ formed per plant x hr) was maximal at approximately the full bloom stage with all maturity groups, irrespective of the calendar date at which the maturity groups attained full bloom. Thus, enzyme activity appeared to be closely associated with physiological growth stage. Genetic variation (up to 1.8 fold) in enzyme activity (seasonal means of µmoles NO-₂ formed per plant x hr) was found within maturity groups II, III, and IV. However, enzyme activity did not correlate with either seed yield or seed protein content.

Key Words: Seed Protein • Enzyme profiles • Soybean Maturity Groups

² Plant Physiologist, U. S. Regional Soybean Laboratory, Plant Science Research Division, Agricultural Research Service, U. S. Department of Agriculture, Urbana, Illinois, Research Technician, and Professor of Agronomy, Illinois Agricultural Experiment Station, Urbana, Illinois, respectively.

Received for publication December 6, 1971.