AGRONOMIC CHARACTERISTICS THAT IDENTIFY HIGH-YIELD, HIGH PROTEIN SOYBEAN GENOTYPES

Seed yield and seed protein content are both heritable traits; however, breeding trials frequently reveal a negative correlation between the two traits. The objectives of this study were to measure and calculate various N-dependent growth characteristics of soybean [Glycine max (L.) Merr.] and to determine their relation to seed yield and seed protein. Well-nodulated plants, each fixing approximately 180 mg N during pod fill, were grown hydroponically in a growth chamber with or without NO3-N during pod filling. Twenty growth-yield characteristics, including yield, harvest index, N harvest index, and Kjeldahl analysis of N2 fixation, were measured or calculated for each of the 384 plants examined. The highest seed yields, approximately 10 g plant-1, and the highest seed N contents, approximately 560 mg plant-1, were obtained when well-nodulated plants were provided some fertilizer-N during pod fill. Except for N content (%) of the dried plant, correlations between each pair of the 20 N-dependent growth-yield characteristics were generally positive. In the absence of fertilizer-N during pod fill, however, N content (%) of the seeds did not correlate with either harvest index or N harvest index, suggesting that insufficient N during pod fill interferes with the orderly mobilization of foliar-N to the developing seeds. New physiological parameters (seed yield merit, N yield merit, and merit of genotype) are proposed for the identification of genetic lines that produce both a high seed yield and a high seed protein content.