Evaluation location: Mississippi, United States
The first-stage screen for N2 fixation tolerance to soil-water deficit was the measurement of petiole ureide levels under well-watered conditions. The entries sampled were accessions in maturity groups V through VIII from the USDA Soybean Germplasm Collection and were grown at Stoneville, MS, on a Bosket fine-sandy loam soil (fine-loamy, mixed thermic, Mollic Hapludalf). Entries were grown in either four-row evaluation plots, four-row seed increase plots, or one-row comparison plots. Approximately 1000 plant introductions, which were blocked by maturity group, were screened each year. These introductions represented nearly 40 different countries but most of the accessions came from China (40%), Japan (21%), Indonesia (12%), S. Korea (5%), India (5%), and USA (5%). Plots were maintained under well-watered conditions and irrigated as necessary. In each of three years (1995, 1996, and 1997), petioles were harvested on one day for all cultivars. The harvest day was selected to be at approximately mid-flowering for many of the genotypes and ranged from R1 to R4. Petioles were harvested from the uppermost, fully-expanded leaves from three plants, placed in coin envelopes, and stored in ice chests for approximately 6 h. Subsequently, the petioles were oven dried at 80.C and finely chopped. Approximately 30 mg of petiole tissue was extracted in 1 mL of 0.2 M NaOH for 30 min at 100.C Ureide concentration was determined with an autoanalyzer based on the method of Van Berkum and Sloger (1983) with the modification that the alkaline hydrolysis step was omitted.