Drought tolerance and yield increase of soybean resulting from improved symbiotic N 2 fixation

Abstract

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 (N 2) is sensitive to even modest soil water deficits. Decline of N 2 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 N 2 fixation to drought. A preliminary physiological screen was used to identify lines that potentially expressed N 2 fixation drought tolerance. One hundred progeny lines derived from a cross between Jackson, a cultivar proven to have N 2 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 N 2 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 N 2 fixation to water deficit.