Mapping quantitative trait loci (QTL) for plant nitrogen isotope ratio (δ 15 N) in soybean

Soybean ( Glycine max (L.) Merr.) meets a large portion of its nitrogen (N) need via biological N 2 fixation, which is highly sensitive to drought stress. Nitrogen isotope ratios between 15 N and 14 N (δ 15 N) can be used as a metric for relative differences among soybean genotypes for N 2 fixation, as δ 15 N is negatively associated with N 2 fixation. This study aimed to dissect the genetic basis of δ 15 N using a mapping population of 196 F 6 -derived recombinant inbred lines developed from a cross between PI 416997 and PI 567201D that was evaluated in multiple environments. There was a wide range of δ 15 N in all environments and narrow-sense heritability for δ 15 N was 35% when estimated across environments. Analysis of variance of δ 15 N showed significant effects of genotype and environment, whereas the genotype × environment interaction was not significant ( P < 0.05). Inclusive composite interval mapping for individual environments identified 10 additive QTLs on seven chromosomes with additive effects ranging from 0.02 to 0.13‰ and that individually explained phenotypic variations from 1.72 to 9.34%. In total, eight QTL × environment interactions were found, and several genomic regions were involved in QTL × QTL interactions that were not identified as additive QTLs. These identified QTLs were co-localized with genomic regions associated with N 2 fixation and other physiological traits identified in previous studies. A search for candidate genes resulted in detection of genes for nodulation and N-metabolism underlying many additive and epistatic QTLs. These identified regions may serve as potential targets for enhancing N 2 fixation in soybean.