Increased nitrogen fixation and remobilization may contribute to higher seed protein without a yield penalty in a soybean introgression line

The development of soybean varieties with higher seed protein concentration has been hindered by a negative correlation between seed protein concentration and yield. Benning HP, a genotype that breaks this tradeoff, contains a high protein allele introgressed into the cultivar Benning. Because seed protein is thought to be limited by N, field and growth chamber experiments were performed to identify the N flux(es) that enable Benning HPs increased seed protein without a yield penalty. When the N source was completely controlled in growth chambers, Benning HP was able to fix more N than its recurrent parent, but this response depended on the strain of the rhizobia symbiont and was not observed at all developmental stages. In the field, Benning HP remobilized N from its leaves at a higher rate during seed fill, but this response was only observed in one of the years studied. These results demonstrate that Benning HP has higher potential for N fixation and N remobilization from vegetative tissue compared to its lower protein recurrent parent, but those traits are not consistently expressed and may depend on environmental and sink control.