Variation in Mobilization of Plant Nitrogen to the Grain in Nodulating and Non‐Nodulating Soybean Genotypes1

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Abstract
Due to the large nitrogen requirement by soybeans (Glycine max L. Merr.), efficient N utilization might conserve plant energy for other metabolic processes. One method of improving efficiency of N utilization would be to have a high total N content in the seeds vs. that in the above‐ground portion of the plant (i.e. a high N index). The objective of this field study was to evaluate a diverse group of 32 nonnodulating lines and nodulating and non‐nodulating near isogenic lines of ‘Clark’ and ‘Harosoy’ for genetic variability in harvest nitrogen index when grown at three N fertilizer rates. In a second year's experiments lines representing extremes in harvest N index were sampled throughout reproductive development and quantity of N in leaflets, petioles, stems and pods was determined. Harvest indices for grain yield and N varied among the genotypes tested. Harvest indices and harvest nitrogen indices were positively correlated with seed yield. A significant positive correlation between harvest and harvest nitrogen indices suggested that certain genotypes are efficient in mobilizing both N and dry matter to the developing seed. Differences in harvest nitrogen index at maturity resulted from differential N mobilization from leaflets, petioles and stems of efficient (high harvest N index) as compared to inefficient (low harvest N index) genotypes. In general stem N characteristics were most diagnostic of harvest N index. At maturity harvest N index was statistically correlated with grams of N in stems, percent N in stems, and percent of maximum N accumulated that was translocated from the stems. Averaged across genotypes adding N fertilizer increased the quantity of whole plant N, grain yield and percent protein in the seed. However harvest N indices were not significantly affected by N fertilization. The ranking of genotypes for harvest and harvest N indices were consistent in different environments, but the inclusion of abscised leaflets and petioles caused some differences in ranking. Of the genotypes evaluated noduating Harosoy had the greatest grain yield and was among the highest in N index.