SynopsisWe studied the effects of oxygen (aerated versus O₂ depleted ∼0.5 mg 1⁻¹ O₂) and nitrate (none versus 10 μmol 1⁻¹) on the ammonium uptake kinetics and adenylate pools in two wetland plants differing in their degree of flood tolerance (Phalaris arundinacea L. and Glyceria maxima (Hartm.) Holmb.). The study was performed as a random block design in a growth chamber. The -uptake kinetics were estimated by using a computerised nonlinear parameter estimation procedure to fit the differential form of a modified Michaelis–Menten model to solution depletion curves. The uptake kinetics for differed between the two species: V_max was significantly higher for P. arundinacea (24.7 to 29.6 μmol h⁻¹ g⁻¹ root dry weight) than for G. maxima (4.6–10.3 μmol h⁻¹ g⁻¹ root dry weight). The concentration at which uptake ceases (C_min) was 0.2 to 0.5 μmol 1⁻¹ for P. arundinacea and significant higher (1.1–2.7 μmol 1⁻¹) for G. maxima.K_m varied between 3.1 and 6.2 μmol 1⁻¹ for P. arundinacea, and 1.6 and 3.0 μmol 1⁻¹ for G. maxima. The different uptake kinetics of the two species reflect the different structure of their root systems: P. arundinacea has an extensive root system consisting of many thin roots whereas G. maxima has fewer but thicker roots. The uptake kinetics also suggest that P. arundinacea is adapted to growing at lower ambient concentrations than G. maxima. Oxygen had no consistent effect on uptake kinetics. However, the plants that had in the nutrient solution as well as had slightly higher V_max values and lower C_min and K_m values than those without . Thus, both species were able to sustain their uptake characteristics at low external O₂ concentrations, probably because of internal aeration through the air-space tissue of the plants. Nitrate deprivation also lowered the energy charge ratio and adenine nucleotide content in roots. The roots recovered quickly from deprivation once was resupplied. The stresses imposed by partially O₂-depleted conditions and lack of nitrate were therefore relatively mild and reversible. It seems that the inherent aerenchyma development under aerated conditions in these species is sufficient to maintain adequate root oxygenation under partially O₂-depleted conditions.