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 hypothe sis that large nodules may help confer drought tolerance because the fracti on 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 defici t, increase phloem water supply to nodules, maintain nodule permeability to O-2, provide sugars to support nodule activity, and supply water for the e xport of ureides from nodules. This hypothesis was evaluated for the cultiv ars 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-w atered and water-deficit conditions. For both cultivars, large nodules main tained 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 di am. For plants of the water-deficit treatment, C-14 concentration of nodule s less than or equal to2 mm diam. was only 1.6 times that of nodules greate r than or equal to4 mm diam. Nodules from the plants of the water-deficit t reatment had a greater C-14 concentration than nodules from the well-watere d treatment for all nodule size classes >2 mm diam. Additionally, C-14 conc entration for all nodule size classes was greater for Jackson than for KS48 95. We conclude that drought tolerance of Jackson is partially due to the a dvantages of large nodules, but that drought tolerance in Jackson also resu lts from an inherently greater supply of photosynthates to nodules.