Nitrogen Nutrition of Field‐grown Soybean Plants: II. Seasonal Variations in Nitrate Reductase, Glutamate Dehydrogenase and Nitrogen Constituents of Plant Parts 1

The goals of this portion of the study were to determine (a) the distribution of N compounds in the aboveground portion of soybean plants and (b) fluctuations in the capacity for N assimilation in the leaves. Soybean plants ( Glycine max (L.) Merr., ‘Harosoy 63’) grown der field conditions were sampled at 14 growth stages for the determination of dry weight, total N, amino N and nitrate N of leaves, stems, and fruits. Nitrate reductase activity and glutamate dehydrogenase activity in leaves were determined at 13 harvest dates. Free amino acid content of leaves, stems, and fruits was determined for one replicate and 13 harvest dates. Amino N was only 2 to 6% of total N in stems and leaves at most harvests indicating that amino acids transported to the tops are rapidly converted to protein. However, up to 40% of the N in stems was in the form of nitrate at the earliest harvests but this proportion fell rapidly to 5% in the flowering stage and to zero in later stages of development. Nitrate reductase content of the leaves was also highest preceding flowering while glutamate dehydrogenase showed the opposite trend, increasing four‐fold from the flowering stage to latter stages of seed development. Seasonal averages indicated that asparagine was the predominant free amino acid in stems and fruits. Asparagine content of leaves decreased approximately 20‐fold and stems 100‐fold from the first to the thirteenth harvest. The N nutrition of the soybean plants in this experiment appeared to be dependent on nitrate assimilation prior to flowering, but strongly dependent on the assimilation of ammonia from N fixation from the flowering stage to maturity.