Host genetic variation in the early nodulation and dinitrogen fixation of soybean

Soybeans [Glycine max (L.) Merr.] grown on soils low in N can exhibit N deficiency during early plant development. Improved early N₂ fixation may alleviate this problem. Our objectives were to (i) examine genetic variation in early nodulation and N₂ fixation of soybean, and (ii) identify traits useful in breeding for early nodulation and N₂ fixation in this crop. Eighty-six soybean lines showed considerable genetic variation in efficiency and speed of nodulation with Bradyrhizobium japonicum strain USDA 110. Eighteen lines were then reevaluated for nodulation efficiency with the B. japonicum strains USDA 123, UMR 161, and NA 6407 for differences in nodule number 5 to 12 d after inoculation (DAI) and for differences in plant dry weight, nodule fresh weight, acetylene reduction activity (ARA), nodule enzyme activity, leghemoglobin concentration, and nodule soluble protein concentrations (SOLP) 10 to 17 DAI. The 86 lines differed in percentage roots nodulated (PRN; 26-93%) and number of nodules above the root tip mark (NARTM; 0.3-3.0), and in uppermost nodule position (-1.7.5 to 19.5 mm). For the 18 lines that were reevaluated, mean PRN and NARTM values were greater for USDA 123 than for USDA 110 (65.1 vs. 57.9%, 1.4 vs. 1.0, respectively). For these lines, correlation analyses showed that PRN correlated with nodule number at 5 (r = 0.69**) but not 12 DAI, while nodule number 12 DAI correlated positively with nodule fresh weight, ARA, enzyme activity, and SOLP 17 DAI. Plant dry weight, nodule fresh weight, and nodule SOLP assayed at 17 DAI are suggested as traits for use in a breeding program to improve early nodulation and N₂ fixation in soybean.