The response and recovery of nitrogen fixation activity in soybean to water deficit at different reproductive developmental stages

Soybean (Glycine max [L.] Merr.) N₂ fixation is a primary plant mechanism responsible for meeting plant-N demand during seed development. Nitrogen fixation is recognized as a drought-sensitive mechanism; however, N₂ fixation response to water deficit and N₂ fixation recovery at different reproductive stages are not well documented. We tested the hypothesis that water deficit during late reproductive stages would inhibit N₂ fixation and lead to the breakdown of essential leaf proteins and an inability to recover N₂ fixation. Acetylene reduction activity (ARA) and N redistribution response to a 5-d drought period at flowering (R2), early seed fill (R5), and late seed fill (R6) were evaluated in one genotype (Hendricks, maturity group 0). Control plants maintained high rates of nodule activity until late seed fill. Plants drought stressed at R2 and R5 recovered ARA after rewatering and in some cases had higher nitrogenase activity than control plants during mid-seed fill. Recovery of ARA on plants stressed at R2 and R5 was associated with higher shoot N concentration than control plants at maturity. Drought stress at R6 reduced ARA, and the inability to recover ARA after stress alleviation at R6 resulted in decreased individual seed mass, which was likely caused by an acceleration of leaf N redistribution and a shorter seed-fill period. Results emphasized the importance of soybean N₂ fixation during late seed development on seed yield and that the ability to recover N₂ fixation following drought is dependent upon crop developmental stage.