NITROGEN UPTAKE AND PARTITIONING IN RESPONSE TO REPRODUCTIVE SINK SIZE OF SOYBEAN

During reproductive development, seeds become a dominant sink for both carbon and nitrogen. To determine how nitrogen uptake and partitionin g by nonnodulated soybean (Glycine mau [L.] Merrill) are affected by a reduced pod load, all pods, ca. half of all pods, and no pods were re moved at the beginning of seed fill from plants growing in flowing sol ution culture containing 1.0 mM NO3-. Dry matter and nitrogen distribu tions within the plants were determined at periodic harvests. Net upta ke rates of NO3- and net CO2 exchange rates of leaves were measured da ily during the subsequent 25-d period of seed fill. Net uptake rates o f NO3- were determined by ion chromatography as depletion from repleni shed solutions. For plants with a full pod load, both NO3- uptake and CO2 exchange rates were maintained throughout the 25-d period of seed fill. With partial and complete depodding, the CO2 exchange rates of t he upper, most photosynthetically active leaves declined during the fi nal 5-10 d of seed fill. Net NO3- uptake rates, particularly by comple tely depodded plants, were slightly enhanced during the initial 10-15 d after depodding until emergence of additional leaves established new sinks for the photosynthate normally partitioned to reproductive grow th in plants with full pod loads. The initially enhanced uptake rates of NO3-, however, were not of sufficient magnitude to contribute to a significant increase in accumulation of nitrogen in the depodded plant s. These results indicate that photosynthetic capacity is sufficient t o support both reproductive growth and nitrogen acquisition when soybe ans are grown nearly without environmental stress.