UREIDE CONCENTRATION OF FIELD-GROWN SOYBEAN IN RESPONSE TO DROUGHT AND THE RELATIONSHIP TO NITROGEN-FIXATION

Ureides accumulate in soybean (Glycine max [L.] Merr.) petioles during drought under greenhouse conditions despite decreased N-2 fixation an d ureide production. Field experiments with water-deficit treatments w ere established to examine the possibility of ureide accumulation unde r natural drought-stress conditions. Tissue ureide concentration of dr ought tolerant 'Jackson' were compared with either 'Hutcheson' or 'Bil oxi'. For mild water-deficit conditions, petiole ureide levels were gr eater than a well-watered treatment at ten sampling dates for Hutcheso n compared to three dates for Jackson. At only two dates were there si gnificant differences between genotypes in petiole ureide concentratio n, 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 con centration than Biloxi throughout the measurement period for both well -watered and water-deficit treatments. Ureide catabolism in leaves dur ing reproductive development was found to be negatively associated wit h 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 fi eld conditions was negatively associated with drought tolerance of N-2 fixation in these cultivars, and that petiole ureide accumulation may result from decreased ureide catabolism.