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CROP PHYSIOLOGY & METABOLISM

Soybean N₂ Fixation Estimates, Ureide Concentration, and Yield Responses to Drought

^a Dep. of Crop, Soil, and Environmental Sci., 1366 W. Altheimer Drive, Univ. of Arkansas, Fayetteville, AR 72704
^b International Crops Research Institute for the Semi-Arid Tropics, Patancheru 502 324 Andhra Pradesh, India
^c USDA-ARS, Univ. of Florida, Agronomy Physiology and Genetics Laboratory, IFAS Building no. 350, 2005 SW 23rd Street, P.O. Box 110965, Gainesville, FL 32611-0965

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

Increasing N₂ fixation tolerance to drought has been hindered by the labor and costs of quantifying N₂ fixation using ¹⁵N methodologies. The relative abundance of ureides (RAU) in plant tissues has been used for estimating N₂ fixation in soybean [Glycine max (L.) Merr.] grown under well-watered conditions, but it has not been evaluated for drought conditions. The present research evaluated the response of N accumulation to N fertilization, the ability of the RAU technique to predict N₂ fixation under drought conditions, and the response of yield to N fertilization under well-watered and drought conditions. Under drought, shoot N accumulation rate during vegetative growth approximately doubled as the amount of N fertilizer was increased from 10 to 200 kg N ha^–1, indicating a greater sensitivity of N₂ fixation to drought than uptake and assimilation of inorganic N. Under well-watered conditions, the relationship between estimates of N₂ fixation made by ¹⁵N-dilution and RAU agreed within 15% of published reports. Under drought conditions, however, this relationship was greatly different (13 to 43%) from published reports. Fertilization with inorganic N in 1 yr increased grain yield 15 to 25% for the drought treatment and 12 to 15% for the well-watered treatment. In a second year, N fertilization increased yield of both drought and well-watered treatments approximately 9%. This research indicates that the RAU technique for estimating N₂ fixation under drought conditions may be invalid without further refinement, that N₂ fixation is more sensitive to drought than the uptake and assimilation of inorganic soil N, and that increasing the tolerance of N₂ fixation to drought would likely result in yield increases.

Abbreviations: DOY, Day of Year • NDA, N derived from atmosphere • NN-Hardee, nonnodulating ‘Hardee’ • ^PNDA, proportion of N derived from atmosphere between harvests • RAU, relative abundance of ureides

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