Remobilization Patterns of C and N in Soybeans with Different Sink-Source Ratios Induced by Various Night Temperatures

The effects of increased sink-source ratios, induced by elevating night temperatures, on remobilization of (14)C-assimilates and N within field-grown soybeans (Glycine max [L.] Merr.) was investigated from preflowering to maturity. Raising the mean minimum night temperature for the entire growing season from 10 (check, uncontrolled) to 16 degrees C increased seed growth without appreciable effect on final leaf area. Increasing this temperature to 24 degrees C increased seed growth and reduced final leaf area. Leaves, stems, petioles, and pods acted as intermediate storage sites for (14)C assimilates. Only plants with higher night temperatures remobilized some of the stored assimilates during the period of rapid seed growth. Even the seeds in the 24 degrees C plants with the largest sink-source ratios did not utilize all the C-assimilates potentially available for remobilization. Nitrogen was readily remobilized from petioles, stems, and pods of all treatments as early as the beginning of seed development, but from the leaves only during late seed-filling. However, only plants with elevated night temperatures tended to remobilize all of the available N from vegetative tissues and pods. We concluded that a larger portion of stored assimilates may be remobilized to the seed if a strong seed sink can be sustained. It also appeared that with increasing sink-source ratios, N shortage might limit seed yield before a lack of C-assimilates would. A proposed model for soybean assimilate demand, distribution, partitioning, and remobilization is presented.