Lower grain nitrogen content of wheat at elevated CO₂ can be improved through post-anthesis NH₄ ⁺ supplement

We test the hypothesis that reduction in grain N concentration under elevated CO₂ concentration (e[CO₂]) is associated with N types (NH₄ ⁺ and NO₃ ⁻) and their ratios. Wheat (Triticum aestivum L. cv. H45) was grown in a glasshouse under two CO₂ concentrations (389 μmol mol⁻¹ and 700 μmol mol⁻¹), supplied with equal amount of N with different ratios of NH₄ ⁺ and NO₃ ⁻: (i) 100% NO₃ ⁻–N; (ii) 50% NO₃ ⁻–N and 50% NH₄ ⁺–N; and (iii) 25% NO₃ ⁻–N and 75% NH₄ ⁺–N. Plant growth, N uptake and partitioning were measured during plant development. Plant biomass and grain yield was increased at e[CO₂] when N was supplied as an equal proportion of NO₃ ⁻ and NH₄ ⁺. Despite the yield increment, grain N concentration was not affected by e[CO₂], in 50% NO₃ ⁻–N treatment. In contrast, grain N concentration decreased in 100% NO₃ ⁻–N and 25% NO₃ ⁻–N treatments. In 50% NO₃ ⁻–N treatment, N uptake during post-anthesis stage (from 69 to 141 days after planting) was significantly stimulated under e[CO₂] compared to 100% NO₃ ⁻–N and 25% NO₃ ⁻–N treatments. We concluded that supplement of N in an equal proportion of NO₃ ⁻ and NH₄ ⁺ which increases post-anthesis N uptake, avoid the reduction of grain N concentration under e[CO₂].