The effects of NH 4+ and NO 3− on growth, resource allocation and nitrogen uptake kinetics of Phragmites australis and Glyceria maxima

Abstract

The effects of NH 4 + or NO 3 − 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 NH 4 +-N cultures than in NO 3 −-N cultures (mean = 66.7 and 57.4 mg g −1 d −1 of NH 4 + and NO 3 − treated plants, respectively). Phragmites and Glyceria had higher S/R ratio in NH 4 + cultures than in NO 3 − 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 NH 4 + treated plants by 16%. The tissue NH 4 + concentration (mean = 1.6 μmol g fresh wt −1) was not affected by N treatment, whereas NO 3 − contents were higher in NO 3 − (mean = 1.5 μmol g fresh wt −1) than in NH 4 + (mean = 0.4 μmol g fresh wt −1) treated plants. In Phragmites, NH 4 + (mean = 1.6 μmol g fresh wt −1) and NO 3 − (mean = 0.2 μmol g fresh wt −1) contents were not affected by the N regime. Species did not differ in NH 4 + (mean = 56.5 μmol g −1 root dry wt h −1) and NO 3 − (mean = 34.5 μmol g −1 root dry wt h −1) maximum uptake rates ( V max), and V max for NH 4 + uptake was not affected by N treatment. The uptake rate of NO 3 − was low in NH 4 + treated plants, and an induction phase for NO 3 − was observed in NH 4 + 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 NH 4 +-N source may participates in the observed success of Glyceria at NH 4 + rich sites, although other factors have to be considered. Higher plasticity of Phragmites toward low nutrient availability may favour this species at oligotrophic sites.