Identification of soybean genotypes with N₂ fixation tolerance to water deficits

Symbiotic N₂ fixation in soybean [Glycine max (L.) Merr.] has been shown to be very sensitive to drying soil. Since this loss of N₂ fixation activity has a negative impact on crop yield under many cropping situations, alleviation of this response would be a major benefit. The objective of this research was to screen a large number of plant introduction lines in an effort to identify soybean germplasm that might have substantially decreased sensitivity of N₂ fixation to water deficits. A three-stage screening process was developed to narrow successively the number of candidate lines for tolerance in this trait. The first-stage screen involved the measurement of petiole ureide levels in a large number of plant introductions (approx. 1000 per year). Approximately 10% of the lines with low petiole ureide levels, which are generally associated with N₂ fixation tolerance, were selected. The second-stage screen involved subjecting field plots of each selected plant introduction to a sustained water deficit of approximately 3 wk and measuring N accumulation during this period. Again, about 10% of these lines were selected for the intensive third-stage screen based directly on an acetylene reduction assay during a cycle of soil drying. From an initial population of about 3000 plant introduction lines, eight lines were identified as having substantial tolerance of N₂ fixation to soil drying. These eight lines are to be used in physiological studies to investigate the basis of their tolerance, and in breeding efforts to incorporate this trait into commercial germplasm.