Submitted to: Journal of Plant Nutrition
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/15/2007
Publication Date: 2/1/2008
Citation: Naegle, E., Kwanyuen, P., Burton, J.W., Carter Jr, T.E., Rufty, T. 2008. Seed Nitrogen Mobilization in Soybean: Effects of Seed Nitrogen Content and External Nitrogen Fertility. Journal of Plant Nutrition, 31:367-379. Interpretive Summary: Soybean breeding programs are developing varieties with higher oil contents. In soybean seed, however, increases in oil content are accompanied by proportional decreases in seed protein. This may adversely affect the seedling by lowering the seed N reserves available for growth. N fertilizer is not commonly applied to soybean fields in the Southeastern USA, and, thus, seedlings are dependent on N reserves from the seed until N2 fixation becomes active, about 4 weeks after germination. Seedlings typically experience N stress during this developmental period. In that regard, under low fertility conditions, lower seed N content is associated with slower seedling growth, decreased leaf initiation and expansion, and reduced ability to respond to an external N supply. In this manuscript, we describe research with soybean seedlings that examines N release from seed having different N contents. Specifically, we attempt to answer the following questions: When soybean seeds have different N contents: 1) Are the patterns of seed protein degradation and N mobilization altered? And 2), is mobilization of seed N responsive to the N status of seedlings, i.e. when seedlings are supplied with external N and the tissue N concentration is higher? The answers provide insights into the physiological changes accompanying reductions in seed protein, and their agronomic implications.
Technical Abstract: Soybean breeding programs have developed genetic lines with relatively low seed protein, which could negatively impact early seedling growth in low fertility conditions that are commonly encountered in the Southeastern USA. In a series of experiments, we investigated seed protein mobilization and its regulation in situ in soybean lines with different seed protein levels. The results showed that rates of N release from cotyledons were much lower with decreasing levels of N in seed. Patterns of proteolysis of the storage proteins glycinin and '-conglycinin and their subunits were not different, but breakdown rates were slower. Seed N release rates increased somewhat when external N was supplied to roots of the developing seedlings, suggesting the involvement of source/sink controls. The effect appeared to be down-stream from proteolysis, as rates of protein breakdown were not altered. The results indicate that low seed protein levels will lead to reduced seedling fitness in low fertility soil conditions unless fertilizer N is applied.