To elucidate the response of oxidative metabolism, triggered by elevated ammonium (NH₄⁺) concentrations, on root growth of wheat seedlings, Yumai 49 (NH₄⁺-tolerant) and Lumai 15 (NH₄⁺-sensitive) cultivars were supplied with either 5.0 mmol L^–1 NH₄⁺-N (EAC) or 5.0 mmol L^–1 NO₃^–-N (CON) under hydroponic conditions.  Root growth in both cultivars was significantly reduced under EAC, and the negative effect was greater in Lumai 15.  EAC enhanced the activities of monodehydroascorbate reductase and dehydroascorbate reductase in the roots of both cultivars, while it decreased ascorbic acid (ASA) content and GDP-mannose pyrophosphorylase (GMPase) activity at the 12th day after treatment in Lumai 15 by 62.0 and 71.4%; and in Yumai 49 by 38.8 and 62.2%, respectively, indicating that the regeneration of ASA was increased, but the biosynthesis of ASA was reduced under EAC treatment.  Moreover, EAC increased DHA/ASA, reactive oxygen species (ROS), and malondialdehyde contents, as well as antioxidant enzyme activities in the roots of both cultivars.  Relatively greater increases in ROS and soluble sugar, and lower antioxidant enzyme activities in Lumai 15 indicate severe disruption of oxidative metabolism when compared to Yumai 49.  Results reveal that the reduction of ASA biosynthesis via decreased GMPase activity under the EAC condition probably acts as a trigger for accumulated ROS and imbalanced redox status, resulting in root growth inhibition during wheat seedling growth stage.  Yumai 49, being an NH₄⁺-tolerant cultivar, had the stronger capacity to protect itself from oxidative stress, which allowed it to retain a lower DHA to ASA ratio by maintaining a better redox homeostasis than could be maintained in the NH₄⁺-sensitive cultivar Lumai 15.

Excessive nitrogen (N) fertilization with a high basal N ratio in wheat can result in lower N use efficiency (NUE) and has led to environmental problems in the Yangtze River Basin, China.  However, wheat requires less N fertilizer at seedling growth stage, and its basal N fertilizer utilization efficiency is relatively low; therefore, reducing the N application rate at the seedling stage and postponing the N fertilization period may be effective for reducing N application and increasing wheat yield and NUE.  A 4-year field experiment was conducted with two cultivars under four N rates (240 kg N ha^–1 (N240),
180 kg N ha^–1 (N180), 150 kg N ha^–1 (N150), and 0 kg N ha^–1 (N0)) and three basal N application stages (seeding (L0), four-leaf stage (L4), and six-leaf stage (L6)) to investigate the effects of reducing the basal N application rate and postponing the basal N fertilization period on grain yield, NUE, and N balance in a soil-wheat system.  There was no significant difference in grain yield between the N180L4 and N240L0 (control) treatments, and the maximum N recovery efficiency and N agronomy efficiency were observed in the N180L4 treatment.  Grain yield and NUE were the highest in the L4 treatment.  The leaf area index, flag leaf photosynthesis rate, flag leaf nitrate reductase and glutamine synthase activities, dry matter accumulation, and N uptake post-jointing under N180L4 did not differ significantly from those under N240L0.  Reduced N application decreased the inorganic N content in the 0–60-cm soil layer, and the inorganic N content of the L6 treatment was higher than those of the L0 and L4 treatments at the same N level.  Surplus N was low under the reduced N rates and delayed basal N application treatments.  Therefore, postponing and reducing basal N fertilization could maintain a high yield and improve NUE by improving the photosynthetic production capacity, promoting N uptake and assimilation, and reducing surplus N in soil-wheat systems.