Soybean nodule size and relationship to nitrogen fixation response to water deficit

Decreased N-2 fixation in response to drought in soybean [Glycine max (L.) Merr.] constrains grain and protein production, but differences exist among cultivars in sensitivity of N-2 fixation to drought. We tested the hypothesis that large nodules may help confer drought tolerance because the fraction of N-2-fixing tissue (i.e., noncortical) is greater in large than small nodules. Consequently, the high energy demand of N-2 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 O-2, 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 O-2 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 (CO2)-C-14, nodules less than or equal to2 mm diam. had approximately 3.5 times the C-14 concentration of nodules greater than or equal to4 mm diam. For plants of the water-deficit treatment, C-14 concentration of nodules less than or equal to2 mm diam. was only 1.6 times that of nodules greater than or equal to4 mm diam. Nodules from the plants of the water-deficit treatment had a greater C-14 concentration than nodules from the well-watered treatment for all nodule size classes >2 mm diam. Additionally, C-14 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.