Soybean nitrogen fixation and nitrogen redistribution response to drought at different reproductive stages

Soybean (Glycine max [L.] Merr.) is an economically important crop in part due to the high protein content of the seed, which requires large amounts of N. During seed development, N accumulation is derived from redistributed N and from N2 fixation. However, the association between N2 fixation and N redistribution across genotypes differing in maturity and in response to drought at different reproductive stages is not well documented. This research evaluated soybean N2 fixation and N remobilization during the reproductive development of different genotypes and maturity groups (MG) isolines in field conditions, and estimated nitrogenase response to drought at different reproductive stages in growth chamber experiments. In field conditions, late MG genotypes had a longer vegetative period, accumulated more biomass prior to seed fill, and had a similar seed-fill period as did early MG genotypes. Larger vegetative N content found in late MG isolines prior to seed development did not increase seed N content. The lack of relationship between redistributed N and seed N, emphasized the importance of N2 fixation supplying N to the seed during seed development. Nitrogen fixation occurred through late seed development under well-watered conditions, and during seed-fill contributed approximately 45% of the N present in the seed. Additionally, during the entire season, N2 fixation contributed approximately 90% of the total N in the seed, regardless of MG. In growth chamber experiments, a short period of drought stress at flowering and early seed fill decreased N concentration in the vegetative tissues, but stressed plants recovered and maintained high nitrogenase activity throughout seed fill. At maturity, plants stressed at flowering and early seed fill had increased shoot N concentration compared to control plants. Soybean drought stressed at late seed fill accelerated leaf N redistribution and plants were unable to increase N2 fixation after rewatering. Our results indicate that during seed development N redistribution and N2 fixation are important sources of N to the seed. Drought stress during late seed growth irreversibly damages nodule activity leading to increased dependence upon remobilized N to meet seed N demands, and results in shorter seed- fill duration.