Beschreibung:
<jats:p>Water deficit in soybean [<jats:italic>Glycine max</jats:italic> (L.) Merr.] results in the accumulation of the products of N<jats:sub>2</jats:sub> fixation (ureides) in shoots, and this may lead to feedback inhibition of N<jats:sub>2</jats:sub> fixation. Manganese is required for ureide degradation in leaves, and it was hypothesized that increased leaf Mn<jats:sup>+2</jats:sup> would alleviate ureide accumulation during drought, lessen feedback inhibition, and prolong N<jats:sub>2</jats:sub> fixation. In a growth chamber experiment, ureides supplied through roots decreased N<jats:sub>2</jats:sub> fixation in well‐watered plants, and a soybean cultivar with demonstrated tolerance of N<jats:sub>2</jats:sub> fixation to water deficit (Jackson) had a lesser concentration of shoot ureides following exogenous ureide application than a cultivar sensitive to water deficit (KS4895). In a greenhouse experiment, N<jats:sub>2</jats:sub> fixation in Jackson under moderate water deficit was not different from the control in the absence or presence of soil‐applied Mn<jats:sup>+2</jats:sup>, whereas N<jats:sub>2</jats:sub> fixation in KS4895 was 30% of the control in the absence of soil‐applied Mn<jats:sup>+2</jats:sup> and 111% of the control in the presence of soil‐applied Mn<jats:sup>+2</jats:sup> Increased N<jats:sub>2</jats:sub> fixation in KS4895 with soil‐applied Mn<jats:sup>+2</jats:sup> under water deficit was associated with decreased shoot ureide concentration. In field experiments, Jackson consistently had lesser shoot ureide concentrations than did KS4895, and enzymatic degradation of ureides was greater for Jackson on one date than for KS4895. We concluded that ureides inhibit N<jats:sub>2</jats:sub> fixation, that genetic variation in the ability to degrade ureides may be important in drought tolerance, and that increased leaf Mn<jats:sup>+2</jats:sup> concentration promotes ureide breakdown and prolongs N<jats:sub>2</jats:sub> fixation under water deficit.</jats:p>