Field evaluation of soybean genotypes selected for enhanced capacity to nodulate and fix nitrogen in the presence of nitrate

Abstract

The development of cultivars of soybean (Glycine max L. Merr.) which are capable of near-maximum levels of N₂ fixation in high-NO ⁻₃ soils remains a high priority in N₂ fixation research. We report a field study to evaluate nodulation and N₂ fixation by 32 genotypes of soybean, selected after two years of glasshouse screening for superior symbiotic activity in the presence of 2.5 mM NO ⁻₃ . The 32 “NO ⁻₃ -tolerant” genotypes and eight others (three commercial “check” cultivars and five “non-fixing” lines) were inoculated withBradyrhizobium japonicum CB 1809 and sown into a black earth soil (fine, montmorillonitic, thermic Udic Pellusterts) which contained high levels of soil NO ⁻₃ (260 kg N ha⁻¹; 0 to 120 cm depth) and which was free of soybean rhizobia. Nitrogen fixation activity was assessed at 89 days after sowing using the relative abundance of ureides in xylem exudate [(ureide-N/ureide-N+NO ⁻₃ -N+α amino-N)×100] as an index of fixation. Plant growth and nodulation were assessed 11 days later. Genotypes 466, 468, 469 and 464, all of Korean origin, showed the highest levels of symbiotic activity. Many of the remaining 28 “tolerant” genotypes nodulated poorly in the field and displayed levels of N₂ fixation (relative ureides) which were equivalent to two of the commercial “check” cultivars, Bragg and Elf. Correlation matrices of the measured parameters revealed highly significant correlations among the indices of nodulation and N₂ fixation and poor correlation between those measurements and plant growth-seed yield. The levels of NO ⁻₃ tolerance, displayed by the four Korean lines, may prove useful in breeding programs which aim to enhance N₂ fixation by soybean in high-NO ⁻₃ soils.