How does Management Increase Soybean Seed Protein? A Mechanistic Approach to Identifying Limitations and Opportunities

A number of cultural management practices like irrigation and fertilization can influence amino acid supply to seeds, but the effectiveness of these practices to actually increase seed protein has been variable. In addition, data from multi-environment trials show inconsistent patterns of management on protein concentration, and these cannot be reproduced by mechanistic simulation models (unlike seed oil concentration). Trials that quantify the effect of management on seed protein are often confounded by an environmental factor, by the size of the sink demand (i.e. the seed itself) for amino acids, and the initial nitrogen supply and/or ability of the crop to establish efficient symbiotic relationships. We propose a set of field and laboratory trials to understand and quantify the maximum potential of current cultural practices to increase seed protein. Field experiments will be conducted in the midsouth delta region (Arkansas), the upper midsouth region (Kentucky), and a northern site (Minnesota). We will manipulate soil N content in the field experiments through the use of a cereal rye cover crop, starting with ambient amounts of soil inorganic nitrogen and a soil depleted of nitrogen. We will use two fertilization treatments—no additional fertilizer and a high rate of inorganic nitrogen applied at the beginning of soybean reproduction (i.e. peak potential for assimilation into seed protein). Lastly, we will assess a number of soybean maturity groups and genotypes that vary according to seed size and nitrogen partitioning potential. In addition to yield and seed protein content, we will take numerous samples to estimate nitrogen fixation rates and nitrogen content in different plant parts—these will be used to assess nitrogen partitioning. We will also utilize an in vitro cotyledon assay that provides a more detailed assessment of nitrogen pathways in the developing plants. Combined with our field experiments, these laboratory samples and analyses will allow us to determine how management and genotype differences influence protein content in soybean seed.