Drought tolerance and yield increase of soybean resulting from improved symbiotic N2 fixation

Drought is by far the most important environmental factor contributing to crop yield loss, especially in soybean [Glycine max (L.) Merr.] where symbiotic fixation of atmospheric nitrogen (N2) is sensitive to even modest soil water deficits. Decline of N2 fixation with soil drying causes yield reductions due to inadequate N for protein production, which is the critical seed product. In this paper, we present a combined physiological and breeding research effort to develop soybean lines that have diminished sensitivity of N2 fixation to drought. A preliminary physiological screen was used to identify lines that potentially expressed N2 fixation drought tolerance. One hundred progeny lines derived from a cross between Jackson, a cultivar proven to have N2 fixation tolerance to drought, and KS4895, a high-yielding line, were tested in the screen. Seventeen lines were identified for subsequent yield trials in moderate- and low-yielding rainfed environments. Two lines, found to have higher yields than commercial checks in these environments were then tested in the greenhouse for their N2 fixation activity in drying soil. Nitrogen fixation activity was found to persist at lower soil water contents than exhibited by the sensitive parent. These two soybean lines offer a genetic resource for increased yields under rainfed conditions as a result of decreased sensitivity of N2 fixation to water deficit.