Evidence that carbon dioxide enrichment alleviates ureide-induced decline of nodule nitrogenase activity

The hypothesis that elevated [CO(2)] alleviates ureide inhibition of N(2)-fixation was tested. Short-term responses of the acetylene reduction assay (ARA), ureide accumulation and total non-structural carbohydrate (TNC) levels were measured following addition of ureide to the nutrient solution of hydroponically grown soybean. The plants were exposed to ambient (360 micromol mol(-1)) or elevated (700 micromol mol(-1)) [CO(2)]. Addition of 5 and 10 mM ureide to the nutrient solution inhibited N(2)-fixation activity under both ambient and elevated [CO(2)] conditions. However, the percentage inhibition following ureide treatment was significantly greater under ambient [CO(2)] as compared with that under elevated [CO(2)]. Under ambient [CO(2)] conditions, ARA was less than that under elevated [CO(2)] 1 d after ureide treatment. Under ambient [CO(2)], the application of ureide resulted in a significant accumulation of ureide in all plant tissues, with the highest concentration increases in the leaves. However, application of exogenous ureide to plants subjected to elevated [CO(2)] did not result in increased ureide concentration in any tissues. TNC concentrations were consistently higher under elevated [CO(2)] compared with those under ambient [CO(2)]. For both [CO(2)] treatments, the application of ureide induced a significant decrease of TNC concentrations in the leaves and nodules. For both leaves and nodules, a negative correlation was observed between TNC and ureide levels. Results indicate that product(s) of ureide catabolism rather than tissue ureide concentration itself are critical in the regulation of N(2)-fixation.