Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/25/1996
Publication Date: N/A
Citation: Interpretive Summary: Nitrogen (N2) is frequently the nutrient that is most limiting for plant growth. Soybean, when grown in symbiosis with Bradyrhizobium bacteria, can obtain sufficient N2 for growth through a process called symbiotic N2 fixation and thus does not need added N2 fertilizer. Nitrogen fixation occurs in small wart like structures on roots termed nodules. However, a frequent problem with soybeans is that during the early stages of growth nodules may be slow to form or may not be efficient resulting in reduced yield and protein. The objectives of this research were: to determine whether there are soybean genotypes with high N2 fixation rates in early growth stage and to identify plant traits that would be useful in breeding soybean for early N2 fixation. Eighty-six soybean genotypes were evaluated for speed and efficiency of early nodulation. These lines were also evaluated for nodule fresh weight, nodule protein, nodule enzyme activity, plant dry matter production, and N2 fixation. Genetic variation existed fo all traits. Early plant dry weight, nodule soluble protein, and nodule fresh weight were identified as the most useful traits to use in selection for early N2 fixation. Use of these selection criteria in a soybean breeding program will allow for the identification of good genotypes with high N2 fixation rates and may result in improved growth and yield.
Technical Abstract: Soybeans (Glycine max (L.) Merr.) can exhibit nitrogen (N) deficiency symptoms during the first 30 to 40 days after planting. Improvement in early N2 fixation may help alleviate this problem. Our objectives were to: 1) examine the genetic variation in the early nodulation and N2 fixation of soybean, and 2) identify traits that might be used in breeding for early nodulation and N2 fixation in this crop. Initial studies showed considerable genetic variation in 86 soybean lines for efficiency and speed of nodulation. Eighteen randomly selected lines were reevaluated for nodulation efficiency with three additional strains of B. japonicum, for differences in nodule number 5 to 12 days 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 concentration (SOLP) 10 to 17 DAI. The 86 lines differed in percent plants nodulated above the root tip mark (RTM, PRN:26 to 93%), number of nodules above the RTM (NARTM:0.3 to 3.0), and uppermost nodule position (UMN:-17.5 to 19.5 mm), while for the 18 lines reevaluated with different strains, 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). The 18 lines also differed in all other traits examined. Correlation analyses showed that PRN correlated with nodule number at five (r=0.69**), but not at 12 DAI. Nodule number at 12 DAI correlated positively with the fresh weight, ARA, enzyme activity, and SOLP of nodules at 17 DAI. Early plant dry weight, nodule fresh weight, and nodule SOLP assayed at 17 DAI are suggested as good candidate traits for use in a breeding program to improve early nodulation and N2 fixation in soybean.