A Possible Relationship Between Shoot N Concentration and the Sensitivity of N₂ Fixation to Drought in Soybean

Abstract

ABSTRACT Nitrogen fixation in soybean [ Glycine max (L.) Merr.] is sensitive to drought, but genotypic differences in sensitivity exist. The establishment of a link between an easily measured phenotypic trait and genotypic differences in drought sensitivity of N 2 fixation is crucial for screening large numbers of genotypes for drought tolerance. Previous work indicated that shoot N concentration of well‐watered soybean was related to differences among genotypes in relative N and biomass accumulation during drought. On the basis of USDA data for 2300 maturity group IV accessions, we requested seed for 175 accessions with good yield potential and low stem lodging scores. These accessions were evaluated under well‐watered conditions in field trials for shoot N concentration during reproductive development and for seed yield. Twelve accessions with good seed yield and extreme shoot N concentrations were evaluated in the growth chamber for acetylene reduction activity (N 2 fixation) in response to soil drying. From the seven accessions selected for low shoot N concentration, we identified two high‐yielding accessions with drought‐tolerant N 2 fixation. Genotypic differences for sensitivity of N 2 fixation to soil drying were strongly correlated with the concentrations of shoot N and shoot ureides under well‐watered conditions and with concentrations of shoot ureides under drought conditions. These data indicate that shoot N concentrations of well‐watered plants may be useful as an initial screening tool for evaluating soybean genotypes for drought‐tolerant N 2 fixation.