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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

View larger version (46K): [in a new window]   Fig. 1. Nitrogen accumulation rate for nonnodulating and nodulating near isolines of ‘Hardee’ under well-watered and drought conditions at Gainesville, FL. Nitrogen was applied as NH4NO3 in three split applications totaling 10, 100, and 200 kg N ha–1 for the low-N, medium-N, and high-N treatments, respectively. Within a genotype, different letters above bars indicate significant differences among means as determined by an LSD (0.05).

View larger version (26K): [in a new window]   Fig. 2. Relative abundance of ureides (RAU) in petioles averaged across four sampling dates vs. the proportion of N derived from the atmosphere (PNDA) between harvest dates, as determined by 15N-dilution for well-watered and drought treatments at Gainesville, FL. The dashed line is the relationship found by Herridge and Peoples (1990) between RAU and NDA for well-watered, vegetative soybean (y = 1.38 + 0.311x + 0.0057x2).

View larger version (27K): [in a new window]   Fig. 3. Rainfall amounts and distribution at Fayetteville, AR, in (A) 1995 and (B) 1996. Crop developmental stages (Fehr and Caviness, 1977) are indicated near the top of each figure.

View larger version (29K): [in a new window]   Fig. 4. Relative abundance of ureides in petiole tissues during (A) 1995 and (B) 1996 at Fayetteville, AR. Treatments were the following: control, no N fertilizer applied; V6, 112 kg N ha–1 at V6; R2, 112 kg N ha–1 at R2; V6&R2, 112 kg N ha–1 at both V6 and at R2. At each measurement date, LSD values (0.05) are shown. Values were averaged across irrigation treatments in both years (irrigation and N-treatment interaction was nonsignificant).