Genetics of nodulation and nitrogen fixation in Brazilian soybean cultivars

The objective of this work was to study the genetics of the nodulation and biological nitrogen fixation (BNF) characteristics of Brazilian soybean cultivars. Four cultivars were identified with different capacities for BNF: J-200 and Bossier (high), Embrapa 20 (medium) and Embrapa 133 (low), and all possible crosses, including reciprocals, were carried out to obtain the F1, F2 and F3 generations. Three experiments were performed simultaneously, under greenhouse conditions, with the restricted set of generations P1, P2, F2 and F3, and plants were evaluated for nodulation (nodule number, NN, and nodule dry weight, NDW) and plant growth (shoot dry weight, SDW). No significant differences between reciprocal and direct effects were observed, therefore all data from F2, as well as from F3 plants were pooled. The frequency distributions for the tested variables in the F3 families were normal, with no evidences of discontinuities, consistent with polygenic inheritance. In the J-200 x Embrapa 133 and Bossier x Embrapa 133 crosses, the significance of the models for means and variances was less frequent, but was eventually observed for NN and NDW. In the other two experiments, there was a predominance of genetic additive (d) and/or genetic additive variance (D) effects for most of the tested variables, except for NDW in the cross of J-200 x Bossier. Genetic dominant effects (h) and/or genetic dominance variance (H) were detected for all variables in the cross Embrapa 20 x Embrapa 133 and for NDW/NN for Bossier x Embrapa 20. Additive x additive epistatic (i) and interaction genotype x microenvironment effects were less important in all experiments. The narrow-sense heritabilities (h(n)(2)) estimates ranged from 39% to 77%, with higher values for NN and NDW in Bossier X Embrapa 20 and for SDW in Embrapa 20 x Embrapa 133; these were high values when compared to other legumes. The prediction of the genetic potential to generate superior inbred lines for nodulation and BNF capacity indicated that selection could be more effective for crosses Bossier x Embrapa 20, and Embrapa 20 x Embrapa 133.