The effects of NH4+ and NO3? on growth,resource allocation and nitrogen uptake kinetics of Phragmites australis and Glyceria maxima

The effects of NH₄⁺ or NO₃^? on growth, resource allocation and nitrogen (N) uptake kinetics of two common helophytes Phragmites australis (Cav.) Trin. ex Steudel and Glyceria maxima (Hartm.) Holmb. were studied in semi steady-state hydroponic cultures. At a steady-state nitrogen availability of 34 μM the growth rate of Phragmites was not affected by the N form (mean RGR = 35.4 mg g^?1 d^?1), whereas the growth rate of Glyceria was 16% higher in NH₄⁺-N cultures than in NO₃^?-N cultures (mean = 66.7 and 57.4 mg g^?1 d^?1 of NH₄⁺ and NO₃^? treated plants, respectively). Phragmites and Glyceria had higher S/R ratio in NH₄⁺ cultures than in NO₃^? cultures, 123.5 and 129.7%, respectively.Species differed in the nitrogen utilisation. In Glyceria, the relative tissue N content was higher than in Phragmites and was increased in NH₄⁺ treated plants by 16%. The tissue NH₄⁺ concentration (mean = 1.6 μmol g fresh wt^?1) was not affected by N treatment, whereas NO₃^? contents were higher in NO₃^? (mean = 1.5 μmol g fresh wt^?1) than in NH₄⁺ (mean = 0.4 μmol g fresh wt^?1) treated plants. In Phragmites, NH₄⁺ (mean = 1.6 μmol g fresh wt^?1) and NO₃^? (mean = 0.2 μmol g fresh wt^?1) contents were not affected by the N regime. Species did not differ in NH₄⁺ (mean = 56.5 μmol g^?1 root dry wt h^?1) and NO₃^? (mean = 34.5 μmol g^?1 root dry wt h^?1) maximum uptake rates (V_max), and V_max for NH₄⁺ uptake was not affected by N treatment. The uptake rate of NO₃^? was low in NH₄⁺ treated plants, and an induction phase for NO₃^? was observed in NH₄⁺ treated Phragmites but not in Glyceria. Phragmites had low K_m (mean = 4.5 μM) and high affinity (10.3 l g^?1 root dry wt h^?1) for both ions compared to Glyceria (K_m = 6.3 μM, affinity = 8.0 l g^?1 root dry wt h^?1). The results showed different plasticity of Phragmites and Glyceria toward N source. The positive response to NH₄⁺-N source may participates in the observed success of Glyceria at NH₄⁺ rich sites, although other factors have to be considered. Higher plasticity of Phragmites toward low nutrient availability may favour this species at oligotrophic sites.