Isolation of Soybean Lines Capable of Nodulation and Nitrogen Fixation under High Levels of Nitrate Supply1
- J. H. Betts and
- D. F. Herridge2
Results from numerous studies indicate that the proportion of soybean [Glycine max (L.) Men.] crop N derived from N2 fixation ranges between 25 and 50% and rarely exceeds 75%. The remainder is taken up as NO−3 from the soil and the net effect of soybean cropping is depletion of the soil N pool. Therefore, a program was commenced to identify genotypes of soybean which were capable of high levels of nodulation and high rates of N2 fixation in the presence of NO−3. In the first year of screening, 489 genotypes of diverse origin were grown in sand, free of Bradyrhizobium japonicum prior to inoculation with effective strain CB 1809 (USDA136), in unreplicated, 14-L free-draining pots in a glasshouse and supplied with nutrients containing either zero or 2.5 mMNO−3. Plants were sampled at R2 for plant growth (shoot mass), for nodulation, and for the proportion of plant N derived from N2 fixation (relative abundance of ureides in xylem exudate and extracts of shoot axes and nodulated roots). From the original 489 genotypes, 66 “NO−3-tolerant”, 9 “NO−3-sensitive” and 12 others were selected for a second year of screening on the basis of an overall ranking index which related nodulation and N2 fixation (relative ureides) activity at 2.5 mMNO−3 to activity at zero NO−3. In year two, treatments were replicated twice; plant culture and sampling procedures were otherwise identical to those used in the first year. Variation in symbiotic activity at high NO−3 supply was again evident with between 5- and 13-fold differences in the nodulation and ureide indices recorded. In both screenings, genotypes of Korean origin ranked first overall and provided three of the top 10 lines. Of the original 19 Korean genotypes, 15 (80%) were screened in year 2 and nine (47%) were identified for future evaluation. Only 5% of the other 470 genotypes were selected as “NO−3-tolerant” after two years of screening. Isolation of genotypes, superior in nodulation and N2 fixation in the presence of NO−3 under both glasshouse and field conditions, should be achieved with further screening in a high-NO−3 field soil.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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