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CROP PHYSIOLOGY & METABOLISM

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

Univ. of Arkansas, Dep. of Crop, Soil, and Environmental Sciences, 1366 W. Altheimer Drive, Fayetteville, AR 72704

* Corresponding author (lpurcell{at}uark.edu)

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.

Abbreviations: ARA, acetylene reduction activity • C_o, oxygen conductance • DAS, days after sowing • FTW, fraction transpirable soil water • FWC, fractional water content • NDW, nodule dry weight • NFW, nodule fresh weight • NTW, nodule turgid weight • O_e, external oxygen concentration • P_o, permeability to oxygen • RNR, root plus nodule respiration • RWC, relative water content • SA, surface area • TW, transpirable water

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Crop Science 2001 41: 991. [Full Text]  

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