Soybean Nodule Size and Relationship to Nitrogen Fixation Response to Water Deficit

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Abstract
Decreased N2 fixation in response to drought in soybean [Glycine max (L.) Merr.] constrains grain and protein production, but differences exist among cultivars in sensitivity of N2 fixation to drought. We tested the hypothesis that large nodules may help confer drought tolerance because the fraction of N2‐fixing tissue (i.e., noncortical) is greater in large than small nodules. Consequently, the high energy demand of N2 fixation may create a greater sink demand by large nodules than small nodules during water deficit, increase phloem water supply to nodules, maintain nodule permeability to O2, provide sugars to support nodule activity, and supply water for the export of ureides from nodules. This hypothesis was evaluated for the cultivars Jackson (drought tolerant) and KS4895 (drought sensitive) in greenhouse and growth chamber experiments. Individual nodule mass and permeability to O2 were 0.65 to 0.70 times greater in Jackson than in KS4895 under well‐watered and water‐deficit conditions. For both cultivars, large nodules maintained a higher relative water content than small nodules across a range of soil‐water deficits. One day after labeling leaves of well‐watered plants with 14CO2, nodules ≤2 mm diam. had approximately 3.5 times the 14C concentration of nodules ≥4 mm diam. For plants of the water‐deficit treatment, 14C concentration of nodules ≤2 mm diam. was only 1.6 times that of nodules ≥4 mm diam. Nodules from the plants of the water‐deficit treatment had a greater 14C concentration than nodules from the well‐watered treatment for all nodule size classes >2 mm diam. Additionally, 14C concentration for all nodule size classes was greater for Jackson than for KS4895. We conclude that drought tolerance of Jackson is partially due to the advantages of large nodules, but that drought tolerance in Jackson also results from an inherently greater supply of photosynthates to nodules.