Changing the Source-Sink Ratio in Soybean Affects the Amount and Quality of Seed Protein.

The soybean [Glycine max (L.) Merr.] crop of the Northern and Western Corn Belt has historically had lower total protein than other United States growing regions.   However, it is not well documented if changes in total protein coincide with changes in protein quality, or the relative abundance of certain amino acids.  The importance of amino acid balance in protein is documented by discrepancies between crude protein value and the actual feed value of soybeans.  Additionally, this change in protein quality may cause error when using proximate measures such as combustion nitrogen to estimate total amino acids.  Field studies were conducted in 2013 and 2014 in southern Minnesota to produce seed with variable protein quantity to investigate these questions.  Treatments included 40% depodding, 80% depodding, 80% shade, 66% defoliation and removal of non-harvest plants.  Plots were harvested at maturity and protein analysis was conduction by two methods.  Amino acids were measured by cation-exchange chromatography coupled with post-column ninhydrin derivatization.  Crude protein was estimated by multiplying combustion nitrogen by the conventional 6.25 nitrogen-to-protein conversion factor.  Comparison of the two protein measures revealed overestimation of the amino acids by crude protein (y = 0.88*x + 3.93).  A correction was made by calculating the actual percent nitrogen in the total amino acids from chromatography and using this unique nitrogen-to-protein conversion factor for each sample.  This improved the approximation of crude protein to total amino acids (y = 0.92*x + 2.47).  Changing percent nitrogen in the soybean protein accounts for this correction and is a result of changes in relative abundance of amino acids including nitrogen-rich amino acids like arginine and lysine.