Genotypic differences in dinitrogen fixation response to NaCl stress in intact and grafted soybean

Soil salinity is a major limitation to legume production. We evaluated specific soybean [Glycine mnu (L.) Merr.] genotypes to identify a salt-tolerant soybean;Bradyrhizobium system, and to determine by grafting experiments if the scion or the root was responsible for salt tolerance. The effects of three levels of salinity (0, 30, and 60 mM NaCl) on nitrogenase activity (acetylene reduction), nodule number, nodule dry matter, and growth of four soybean genotypes (Williams 82, PI416937, DR-1 [an Egyptian cultivar], and NOD1-3 [a hypernodulating mutant selected from Williams]) were investigated in hydroponic cultures and growth chamber environments. Salt stress significantly inhibited nitrogenase activity, nodule number, and dry matter accumulation per plant of all four cultivars. The detrimental effects of salinity on nodulation parameters and dry matter accumulation were most pronounced for NOD1-3 and Williams 82, intermediate for DR-1, and less marked for PI 416937. Self-grafted NOD1-3 plants showed 50 to 62% inhibition in nodulation responses (activity, number, mass) while grafting of PI 416937 scions to NOD1-3 roots resulted in less than 7% inhibition by salt. Nodule number on PI416937 roots was greater when grafted to NOD1-3 scions (relative to self-grafted PI 416937 plants), confirming a shoot role in autoregulation of nodule number. Conclusions are that shoot factors are of primary importance in determining salt-tolerance of the PI 416937 genotype and that hypernodulation expression in the mutant is negatively affected by salt treatment.