Ureide concentration of field‐grown soybean in response to drought and the relationship to nitrogen fixation

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Abstract
Ureides accumulate in soybean (Glycine max [L.] Merr.) petioles during drought under greenhouse conditions despite decreased N2 fixation and ureide production. Field experiments with water‐deficit treatments were established to examine the possibility of ureide accumulation under natural drought‐stress conditions. Tissue ureide concentration of drought tolerant ‘Jackson’ were compared with either ‘Hutcheson’ or ‘Biloxi’. For mild water‐deficit conditions, petiole ureide levels were greater than a well‐watered treatment at ten sampling dates for Hutcheson compared to three dates for Jackson. At only two dates were there significant differences between genotypes in petiole ureide concentration, in which case petiole ureide concentration of Hutcheson was greater than Jackson. Under more severe water‐deficit conditions, there were greater and more consistent increases in petiole ureide concentration for Jackson and Biloxi. Jackson, however, had lower petiole ureide concentration than Biloxi throughout the measurement period for both well‐watered and water‐deficit treatments. Ureide catabolism in leaves during reproductive development was found to be negatively associated with petiole ureide concentration of Hutcheson (r=‐0.37, P=0.01), but not in Jackson (r=‐0.12, P=0.44). These results indicated that increased ureide accumulation in response to water deficits experienced under field conditions was negatively associated with drought tolerance of N2 fixation in these cultivars, and that petiole ureide accumulation may result from decreased ureide catabolism.