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 mu M 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 mu mol g fresh wt(-1)) was not affected by N treatment, whereas NO3- contents were higher in NO3- (mean = 1.5 mu mol g fresh wt(-1)) than in NH4+ (mean = 0.4 mu mol g fresh wt(-1)) treated plants. In Phragmites, NH4+ (mean = 1.6 mu mol g fresh wt(-1)) and NO3- (mean = 0.2 mu mol g fresh wt(-1)) contents were not affected by the N regime. Species did not differ in NH4+ (mean = 56.5 mu mol g(-1) root dry wt h(-1)) and NO3- (mean = 34.5 mu mol g(-1) root dry wt h(-1)) maximum uptake rates (V-max), and V 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 (mean = 4.5 mu M) and high affinity (10.31 g(-1) root dry wt h(-1)) for both ions compared to Glyceria (K-m = 6.3 mu M, affinity = 8.01 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. (c) 2005 Elsevier B.V. All rights reserved.