Annals of Botany

The hypothesis that elevated [CO₂] alleviates ureide inhibition of N₂-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 µmol mol^-1) or elevated (700 µmol mol^-1) [CO₂]. Addition of 5 and 10 mM ureide to the nutrient solution inhibited N₂-fixation activity under both ambient and elevated [CO₂] conditions. However, the percentage inhibition following ureide treatment was significantly greater under ambient [CO₂] as compared with that under elevated [CO₂]. Under ambient [CO₂] conditions, ARA was less than that under elevated [CO₂] 1 d after ureide treatment. Under ambient [CO₂], 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 [CO₂] did not result in increased ureide concentration in any tissues. TNC concentrations were consistently higher under elevated [CO₂] compared with those under ambient [CO₂]. For both [CO₂] 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 N₂-fixation.