Growth in elevated CO2 often leads to decreased plant nitrogen contents and down-regulation of photosynthetic capacity. Here, we investigated whether elevated CO2 limits nitrogen uptake when nutrient movement to roots is unrestricted, and the dependence of this limitation on nitrogen supply and plant development in durum wheat (Triticum durum Desf.). Plants were grown hydroponically at two N supplies and ambient and elevated CO2 concentrations. Elevated CO2 decreased nitrate uptake per unit root mass with low N supply at early grain filling, but not at anthesis. This decrease was not associated with higher nitrate or amino acid, or lower non-structural carbohydrate contents in roots. At anthesis, elevated CO2 decreased the nitrogen content of roots with both levels of N and that of aboveground organs with high N. With low N, elevated CO2 increased N allocation to aboveground plant organs and nitrogen concentration per unit flag leaf area at anthesis, and per unit aboveground dry mass at both growth stages. The results from the hydroponic experiment suggest that elevated CO2 restricts nitrate uptake late in development, high N supply overriding this restriction. Increased nitrogen allocation to young leaves at low N supply could alleviate photosynthetic acclimation to elevated CO2.
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