2006 Annual Report 4d.Progress report. This report serves to document research conducted under a reimbursable agreement between ARS and the United Soybean Board. Additional details of research can be found in the report for the parent project 3611-21000-020-00D, Identifying and Manipulating Determinants of Photosynthate Production and Partitioning. The molecular mechanisms that control soybean seed protein content are not well understood. In order to examine the role of amino acid availability, free pools of amino acids were measured at different points in the transport pathway in developing seeds of two soybean cultivars differing in protein content at maturity. The two cultivars had similar amino acid pools in the seed coat and apoplast space, but differed in the concentration of amide amino acids within the developing seeds. The results suggest that the high seed protein phenotype may be related to the ability of seeds to actively accumulate amino acids from the apoplast fluid.