Soybean Nitrogen Fixation and Nitrogen Remobilization at Different Reproductive Stages and Water Regimes.

Soybean (Glycine max [L.] Merr.) N₂ fixation during reproductive development is an important process in supplying nitrogen demand. Nitrogen fixation is recognized as a drought sensitive mechanism; however, N₂ fixation response to drought at different reproductive stages is not well documented. We tested the hypothesis that drought during late reproductive stages will cause irreparable damage to N₂ fixation due to the breakdown of essential leaf proteins and the inability of N₂ fixation to recover. In the present work, we   evaluated N₂ fixation in soybean (cv. Hendricks, MG 0) during different reproductive stages under well-watered and drought conditions in growth chamber experiments using a non-destructive acetylene reduction assay (ARA) through the entire plant cycle. Treatments included a moderate drought stress (40% of relative transpiration) for 5 days at beginning of flowering (R1), early seed fill (R5), and late seed-fill (R6) and a well-watered control treatment. Under well-watered conditions, ARA peaked at mid seed-fill and significantly decreased after full-seed stage. At the end of the drought period normalized ARA was reduced to about 40%, regardless of the development stage. However, 5 days after plants were rewatered, ARA had completely recovered for the R2 and R5 treatments. Drought stress at R6 stage had only slight recovery after rewatering. Plants exposed to water deficit during early reproductive stages and then watered during late reproductive stages had prolonged seed-fill duration and maintained high nitrogenase activity during late reproductive stages. Drought stress at late seed-fill, decreased the seed-fill period and, therefore, decreased seed size and seed yield.